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9053 



Bureau of Mines Information Circular/1985 



Ground Control Instrumentation 



A Manual for the Mining Industry 



By Eric R. Bauer 




UNITED STATES DEPARTMENT OF THE INTERIOR 



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Information Circular 9053 



Ground Control Instrumentation 

A Manual for the Mining Industry 



By Eric R. Bauer 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



1 






n 5 



W 




Library of Congress Cataloging in Publication Data: 



Bauer, Eric R 

Ground control instrumentation. 

(Bureau of Mines information circular ; 9053) 

Bibliography. 

Supt. of Docs, no.: I 28.27: 9053. 

1. Ground control (Mining)— Instruments. 2. Rock deformation— Mea- 
surement. I. United States. Bureau of Mines. II. Title. III. Series: Infor- 
mation circular (United States. Bureau of Mines) ; 9053. 



TN295.U4 [TN288] 622s [622'. 2] 



85-600140 









^ 



CONTENTS 

Page 



i Abstract 1 

c Introduction 2 



Acknowledgment 2 

Instrument selection guidelines 2 

Chapter 1. — Convergence measurements..... 4 

Introduction 4 

Ground movement indicators 4 

Instrument selection 4 

Description of individual instruments 6 

Glowlarm 6 

Guardian Angel 7 

Horizontal roof strain indicator 9 

Infrared scanner 10 

Infrared thermometer 11 

Plumb bob 12 

Spider roof monitor 13 

SRC closure rate instrument 14 

Tape extensometer 15 

Tube extensometer 16 

Visual roof sag bolt 17 

References 19 

Chapter 2. — Strata separation measurements 20 

Introduction 20 

Ground movement indicators 20 

Instrument selection 20 

Description of individual instruments 21 

Borehole extensometer 21 

Simple weighted bed separation indicator 22 

Stratascope 23 

References 25 

Chapter 3. — Lateral roof movement measurements 25 

Introduction 25 

Ground movement indicators 25 

Instrument selection 25 

Description of individual instruments 26 

Plumb bob 26 

Stratascope 27 

Chapter 4. — Stress measurements 27 

Introduction 27 

Ground movement indicators 28 

Instrument selection 28 

Description of individual instruments 29 

Borehole deformation gauge 29 

Borehole inclusion stressmeter 31 

Borehole-mount strain gauge 33 

CSIR strain gauge strain cell (Doorstopper) 34 

CSIR triaxial strain cell 35 

CSIR0 hollow inclusion stress cell 36 

Cylindrical borehole pressure cell 37 

Flat borehole pressure cell 38 

Flatjack 40 

Mechanical strain gauge 41 



11 



CONTENTS — Continued 



Page 



Surface-mount photoelastic gauge 42 

Surface-mount strain gauge 43 

Surface rosette undercoring 45 

Vibrating wire stressmeter 46 

References 47 

Chapter 5. — Support load measurements 48 

Introduction 48 

Ground movement indicators 48 

Instrument selection 48 

Description of individual instruments 49 

Gloetzl pressure cell 49 

Powered-support pressure recorder 50 

Prop load cell 51 

Roof bolt load cell 53 

Roof bolt U-cell 54 

Surface-mount photoelastic gauge 55 

Surface-mount strain gauge 55 

Torque wrench 55 

References 56 

Discussion 57 

Bibliography 58 

Appendix A. — Case studies 60 

Appendix B. — Instrument suppliers 66 

ILLUSTRATIONS 

1. Instrument selection worksheet for ground control measurements 3 

2. Type of movement measurable by each convergence measuring instrument 5 

3. Cost range of purchase or fabrication of convergence measuring 

instruments 5 

4. Range of technical ability required for installation, monitoring, and 

data interpretation of convergence measuring instruments 5 

5. Type of measured data obtained from convergence measuring instruments.... 6 

6. Installed Glowlarm 6 

7. Method for measuring bend of Glowlarm at installation 7 

8. Guardian Angel 8 

9 . Cutaway view of roof and installed Guardian Angel 8 

10. Cross section of a typical coal mine opening illustrating the HORSI 

principle 9 

11. Horizontal strain measuring apparatus 10 

12. Infrared scanner 11 

13. Typical digital-readout infrared thermometer 12 

,14. Plumb bob setup for roof convergence measurement 13 

15. The Spider roof monitor 14 

16. Installation diagram for the Spider roof monitor 14 

17. Side view of tape extensometer 15 

18. Top view of tape extensometer 15 

19. Index mark alignment for correct reading of tape extensometer 16 

20. Dial-gauge tube extensometer 17 

21. Visual roof sag bolt at installation 18 

22. Visual roof sag bolt after convergence movements 19 



ILLUSTRATIONS— Continued 



111 



Page 



23. Cost range of purchase or fabrication of strata separation measuring 

instruments 21 

24. Range of technical ability required for installation, monitoring, and 

data interpretation of strata separation measuring instruments 21 

25. Type of measured data obtained from strata separation measuring 

instruments 21 

26. Borehole extensometer 21 

27. Simple weighted bed separation indicator 23 

28. Fiberoptic stratascope, battery pack, camera, and attachment 24 

29. Stratascope setup for roof observation 24 

30. Cost range of purchase or fabrication of lateral roof movement measuring 

instruments 26 

31. Range of technical ability required for installation, monitoring, and 

data interpretation of lateral roof movement measuring instruments 26 

32. Type of measured data obtained from lateral roof movement measuring 

instruments 26 

33. Plumb bob setup for detecting lateral roof movement 27 

34. Cost range of purchase or fabrication of stress measuring instruments.... 28 

35. Range of technical ability required for installation, monitoring, and 

data interpretation of stress measuring instruments 29 

36. Type of measured data obtained from stress measuring instruments 29 

37. Three-component borehole deformation gauge 30 

38. Cross section through a borehole showing borehole deformation gauge after 

overcoring 31 

39. Recommended borehole configurations for complete, three-dimensional, 

state-of-stress determination 31 

40. Cross section of stressmeter 32 

41. Stressmeter and tapered sleeve into which it fits 32 

42. Cross-sectional view of a borehole showing installed photoelastic 

stressmeter 32 

43. Typical installation of a cylindrical borehole pressure cell 37 

44. Steps in fabrication of an encapsulated flat borehole pressure cell 39 

45. Flatjack pressure cell 40 

46. Mechanical strain gauge 41 

47. Typical measurement setup for mechanical strain gauge showing measuring 

points and stress relief holes 41 

48. Example of photoelastic fringe patterns displayed by a surface-mount 

photoelastic gauge 43 

49. Vibrating wire surface-mount strain gauge 44 

50. Surface-mount strain gauges showing stress relief using large overcoring 

bit 44 

51 . Vibrating wire stressmeter 46 

52. Cost range of purchase or fabrication of support load measuring 

instruments 48 

53. Range of technical ability required for installation, monitoring, and 

data interpretation of support load measuring instruments 49 

54. Type of measured data obtained from support load measuring instruments... 49 

55. Powered-support pressure recorder 50 

56. Chart of hydraulic pressures in longwall roof supports during normal 

operation 51 

57. Prop load cell (strain gauge design) 52 



IV 



ILLUSTRATIONS — Cont inued 



Page 



58. Photoelastic prop load cell and readout equipment 52 

59. Typical roof bolt load cells (strain gauge design) 54 

60. Cross-sectional view of a spring-and-disk roof bolt load cell 54 

6 1 . Steps in fabrication of a roof bolt U-cell 55 

62. Torque wrench 56 

A-l. Typical gauge station 61 

A-2. Average deflection, by location 61 

A-3. Instrumentation plan for each array 62 

A-4. Amount and direction of lateral roof movement found in holes 20 through 

29 63 

A-5. Shortwall section 7 right 64 

A-6. Pressure changes recorded in pillars and shortwall panels during mining. . 65 













| 




UNIT OF MEASURE 


ABBREVIATIONS USED 


IN 


THIS REPORT 




°F 


degree Fahrenheit 


min 




minute 




ft 


foot 


yin 




microinch 




ft-lbf 


foot pound 


yin/in 




microinch per inch 




h 


hour 


pet 




percent 




in 


inch 


psi 




pound per square inch 




in2 
in 3 
in/in 


square inch 
cubic inch 
inch per inch 


psi/min 
V 




pound per square inch 
per minute 

volt 




lb 


pound 











GROUND CONTROL INSTRUMENTATION 
A Manual for the Mining Industry 

By Eric R. Bauer 1 



ABSTRACT 

This Bureau of Mines manual is intended to provide a better under- 
standing of ground movement and the technology available for measuring 
it. The manual deals with convergence, strata separation, lateral roof 
movement, stress, and support load in underground mines. The instru- 
ments that measure these ground control parameters are described in de- 
tail. Step-by-step procedures for selecting the appropriate instrument 
are presented, which consider such factors as approximate cost, instal- 
lation procedures, data collection, and data interpretation; and an in- 
strument selection worksheet is provided to facilitate the instrument 
selection process. Actual instrument case studies and a list of instru- 
ment suppliers are presented in the appendixes. 



'Mining Engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Ground control is the control of the 
immediate area surrounding an underground 
excavation, including the stability of 
the mine roof, ribs, and floor. Since 
ground instability can result in unsafe 
mining conditions, it is vitally impor- 
tant that mine operators have effective 
means of controlling the mine area. To 
apply ground control techniques effec- 
tively, it is essential to ascertain the 
type of movement involved and the extent 
of the hazard created. 

Each instrument available for measuring 
ground movements has particular charac- 
teristics, and the instrument to be used 
in each situation must be carefully se- 
lected to achieve optimum results. This 



manual is designed to assist mine opera- 
tors in making such selections. Opera- 
tional features of the instruments are 
included only as an aid to selection. 
Manufacturers of the devices should be 
consulted for instructions in their use. 
Additional information can be obtained 
from instrument suppliers and from 
sources listed in the references and bib- 
liography for effective instrumentation 
use. 

It is hoped that this instrumentation 
manual will improve the understanding of 
ground control movements and provide the 
information needed for efficient instru- 
ment selection. 



ACKNOWLEDGMENT 

The author wishes to express his appre- Pittsburgh Research Center, for his pre- 

ciation to Steven C. Stehney, mining liminary input and his assistance during 

engineer, now at Jim Walter Resources, the initial literature search, 
and formerly with the Bureau of Mines, 

INSTRUMENT SELECTION GUIDELINES 



Efficient instrument selection depends 
on the mine operator's (user's) ability 
to define the requirements and parameters 
of each ground control problem. At times 
this may be rather difficult, but even an 
educated estimate can provide adequate 
guidelines for choosing the appropriate 
instruments. As the requirements and 
parameters are defined, they should be 
listed on an instrument selection work- 
sheet (fig. 1). 

The first step is to identify the 
ground control movement that is occurring 
or needs investigation. This can be con- 
vergence (roof sag, rib failure, floor 
heave), separation of mine roof strata, 
lateral roof movement, stress, or support 
load. When the movement has been iden- 
tified, it should be listed in the "move- 
ment occurring" column. 

The next step is to determine the 
controlling parameters, namely cost, 



technical aspects, and data requirements. 
The cost parameter is the amount of money 
available for instrument purchases. The 
technical aspects parameter deals with 
the expertise of the people who will in- 
stall and monitor the instrument and ana- 
lyze the data. This expertise is desig- 
nated as slight (no technical background 
or previous instrumentation use), moder- 
ate (some technical or engineering back- 
ground and/or previous instrumentation 
use) , or extensive (engineering degree 
and/or extensive instrumentation use). 
The data requirements parameter is the 
type and detail of data desired. Data 
can be either visual, simple numerical, 
detailed numerical, or a combination of 
these. As these controlling parameters 
are defined, they should be listed on the 
worksheet in the "desired elements" 
column. 



The steps given in each chapter should 
then be followed to make a preliminary 
choice of the instrument(s ) best suited 
to the ground control problem being 
investigated. To assist mine operators 
to make the final instrument selection, 
individual instruments are discussed 
in terms of principle and application, 
availability, description, installation 



and operation, data collection, and data 
interpretation. 2 



"2 



Not all of the instruments described 



are commercially available at this time. 
For those that are, suppliers are listed 
in appendix B. For the others, instruc- 
tions for in-house assembly are given 
where possible. 



INSTRUMENT SELECTION WORKSHEET 



Requirements 


Movement occurring 
or 

measurement desired 


Instruments available 






1. 
2. 




Type of 
ground control 




3. 

4. 




problem 




5. 
6. 
7. 




Controlling 
parameters 


Desired 
elements 


Instruments available 


Cost 




1. 
2. 
3. 




Technical aspects 




1. 
2. 
3. 




Data required 




1. 
2. 
3. 





Instruments satisfying all the above requirements and parameters: 



2. 
3. 



FIGURE 1. - Instrument selection worksheet for ground control measurements. 



CHAPTER 1.— CONVERGENCE MEASUREMENTS 



INTRODUCTION 

Convergence is the vertical closure be- 
tween roof and floor. It is also the 
horizontal closure between two parallel 
ribs. It involves three distinct move- 
ments: roof sag, rib failure, and floor 
heave. Roof sag, the downward movement 
of the immediate roof, occurs after the 
coal is mined and is due to the weights 
of the immediate roof and overburden. 
Rib failure — the spalling, lateral expan- 
sion, or shear failure of the pillars — is 
due to excessive loading of insufficient- 
size pillars by the overburden. Floor 
heave, the upward movement of the floor, 
results from the combination of small 
pillars and soft floor; overburden weight 
pushes the pillars downward and this 
pushes the soft floor sideways and 
upward. 

Convergence measurements are an impor- 
tant investigative tool that permits op- 
erators to detect rib, roof, and floor 
movements before they become major ground 
control problems, to analyze ground move- 
ments and prevent further occurrences, 
and to plan changes in mine design. 
Early detection of hazardous ground con- 
ditions can result in increased safety 
and production. 

GROUND MOVEMENT INDICATORS 

Ground movements have specific charac- 
teristics that serve as indicators. Un- 
fortunately, some indicators are common 
to several types of movement. In some 
cases , determining which movement is oc- 
curring can be difficult, amounting at 
times to no more than an educated esti- 
mate. The following list of convergence 
movements and their indicators can help 
to identify the type of movement that is 
occurring. 

Roof sag indicators: 

• Visible closure or convergence of 

entry. 

• Tension cracks in middle of roof 

span. 

• Increase in water dripping from 

cracks in the roof. 



• Bent or broken roof supports. 

• Falls of large blocks of rock. 

• Shear cutters in root along rib 

line. 

Rib failure indicators: 

• Sloughing of ribs at top, center, 

or bottom. 

• Cracks developing in ribs. 

• Bumps or bursts from ribs. 

Floor heave indicators: 

• Fractures developing in floor 

along center line of roadway. 

• Visible closure of entry. 

• Unevenness developing in floor. 

• Excessive seepage of water from 

floor. 

• Sloughing of pillars at floor line 

only. 

• Fractures developing in floor 

along rib line. 

INSTRUMENT SELECTION 

As requirements and parameters are 
defined, they should be listed on an 
instrument selection worksheet (fig. 
1). The convergence movement (roof 
sag, rib failure, or floor heave) 
that is occurring or needs investiga- 
tion should be listed in the "move- 
ment occurring" column. The control- 
ling parameters, as defined in the sec- 
tion "Instrument Selection Guidelines," 
should be listed in the "desired ele- 
ments" column. 

Once the requirements and parame- 
ters have been defined and listed on 
the worksheet, the user should refer 
to figures 2 through 5 and follow 
steps 1 through 6 to make a preliminary 
choice of the instrument(s) best suited 
to the ground control problem being 
investigated. 

Step 1: From figure 2, choose the 
instruments that satisfy the "movement 
occurring" requirement of the work- 
sheet. List these instruments on the 
worksheet under "instruments availa- 
ble," across from the "movement occur- 
ring" parameter. 



Instrument 


Type of convergence measurable 


Roof sag 


Rib failure 


Floor heave 


Glowlarm 


• 


o 


o 


Guardian Angel 


• 


o 




Horizontal roof 
strain indicator 


o 


o 




Infrared scanner 


o 


o 




Infrared 
thermometer 


o 


o 




Plumb bob 


• 




o 


Spider roof 
monitor 


o 






SRC closure rate 
instrument 


o 


o 


Tape extensometer ^B ^B 


o 


Tube extensometer 


• 




o 


Visual roof ^^k 
sag bolt ^B 


o 





Car 



KEY 



Best suited to measure 



FIGURE 2. - Type of movement measurable by 
each convergence measuring instrument. 



Step 2: From figure 3, choose the in- 
struments that satisfy the cost parameter 
previously determined. List them on the 
worksheet under "instruments available," 
across from the cost parameter. 

Step 3: From figure 4, choose the in- 
struments that satisfy the technical as- 
pects parameter. List them under "in- 
struments available," across from the 
technical aspects parameter. 

Step 4: From figure 5, choose the in- 
struments that satisfy the data require- 
ment parameter listed on the worksheet. 
List them under "instruments availa- 
ble," across from the data requirement 
parameter. 

Step 5: Determine from the "instru- 
ments available" column of the worksheet 
the instruments that satisfy all of 
the requirements and parameters. List 
these instruments at the bottom of the 



Instrument 


Cost of purchase or fabrication 


Dollars 

10 50 100 250 500 1,000 2,000 10,000 

1 . . . 1 . 1 . . 1 . . 1 , . , , 1 , 1 ,1 


Glowlarm 


□ 


Guardian Angel 


□ 


Horizontal roof 
strain indicator 


□ 


Infrared scanner 





Infrared 
Thermometer 




, 1 1 




Plumb bob 




1 1 




Spider roof 
monitor 


□ 


SRC closure rate 
instrument 




d Z] 




Tape 
extensometer 




1 1 




Tube 
extensometer 




1 1 




Visual roof 
sag bolt 


□ 





c 



KEY 

Cost range 



IGURE 3. - Cost range of purchase or fabrica- 
tion of convergence measuring instruments. 



Instrument 


Range of technical ability required 


Slight 


Moderate 


Extensive 


Glowlarm 










^w^^ 








Guardian Angel 










^^w^^ 


K ww\ 








\ww\\ 






Horizontal roof 
strain indicator 


^^ mm 














.www 


\\S\\VJ 






Infrared scanner 






















Infrared 
thermometer 




1 








l\\ W 


sWWJ 








Plumb bob 










a^^wt 




Spider roof 
monitor 










^R^wr 




SRC closure rate 

instrument 










^R^^w 




Tape extensometer 














Tube extensometer 















Visual roof 
sag bolt 














i^X\\.\\\\\\yk\Y\\> 



3 Installation 



KEY 

Monitoring 



iwwi Data interpretation 



FIGURE 4. - Range of technical ability required 
for installation, monitoring, and data interpretation 
of convergence measuring instruments. 



Instrument 


Type of measured data obtained 


Visual 


Simple numerical 


Detailed numerical 


Glowlarm 


o 


o 




Guardian Angel 


o 


o 




Horizontal roof 
strain indicator 






o 


Infrared scanner 


o 






Infrared 
thermometer 




o 




Plumb bob 


o 


o 




Spider roof 
monitor 


o 






SRC closure rate 
instrument 


o 


o 




Tape extensometer 




o 




Tube extensometer 


o 


o 




Visual roof 
sag bolt 


o 







KEY 
O Data obtainable 



FIGURE 5. - Type of measured data obtained 
from convergence measuring instruments. 




Scale, in 

FIGURE 6. - Installed Glowlarm (1). 



worksheet. They represent the best pos- 
sible choices, relative to need, for the 
ground control problem being investi- 
gated. If no instruments satisfy all the 
requirements and parameters, it may be 
necessary to change the parameters or to 
choose the instruments that satisfy the 
most requirements and parameters. 

Step 6: At this point, it is up to the 
user to make the final decision as to the 
most suitable instrument(s) , based on the 
following detailed descriptions. 

DESCRIPTION OF INDIVIDUAL INSTRUMENTS 

Glowlarm^ 

Principle and Application 

The Glowlarm is a device that gives 
visible warning of impending failure of 
roof, rib, or floor (1_). 4 It detects 
roof sag, rib expansion, and floor heave 
by "lighting up" when movement occurs. 

Availability 

It is available from Glowlarm Rock Fall 
Warning Devices. Cost is approximately 
$10 per unit. 5 

Description 

The Glowlarm warning device consists of 
a flexible see-through plastic tube about 
6 in long and 0.5 in. in diam, which is 
held in place by an anchor and stainless 
steel wire (fig. 6). It contains two 
liquids separated by an inner glass tube. 
It is the mixing of these two liquids, 
after the glass tube breaks because of 
ground movement, that produces the bright 
yellow warning light. This light per- 
sists for 24 h. 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this chapter. 

^Instrumentation costs are the best 
available estimates based on manufac- 
turers' information at the time the re- 
search was completed. 



Installation 

Step 1: Drill a hole in the roof or 
rib to a stable zone of rock at a depth 
greater than the anchorage zone of the 
'roof bolts being used. 

Step 2: Push the anchor, with wire 
attached, to the end of the hole and set 
by rapping it sharply with a stick. 

Step 3: Wrap wire around the Glowlarm 
and tighten it so that the Glowlarm has 
the desired bend. The amount of bend at 
installation determines the amount of 
movement to be detected. The larger the 
bend the less movement needed to break 
the glass tube, allowing the instrument 
to light up. The bend can be measured 
with a straightedge and rule (fig. 7). 

Step 4: Trim off the excess wire. The 
Glowlarm is now ready to monitor ground 
movements . 



These observations should be made dur- 
ing routine travel throughout the mine. 
A glowing instrument should be report- 
ed immediately to the appropriate mine 
personnel, who should initiate corrective 
action as soon as possible. 

Data Interpretation 

A glowing instrument means that some 
convergence has taken place; however, the 
amount of movement that indicates unsta- 
ble conditions varies from mine to mine. 
Only through experimentation with this 
instrument can mine personnel become 
knowledgeable as to the amount of bend 
required at installation. 

Guardian Angel 

Principle and Application 



Data Collection 

The Glowlarm should be observed at 
least once every 24 h, since this is 
the life span of the warning light. 




R lie 



Scole. in 



FIGURE 7. - Method for measuring bend of 
Glowlarm at installation. 



The Guardian Angel is a device for mea- 
suring and monitoring roof sag (2^). A 
reflector flag drops when the preset 
amount of movement is reached, to signal 
that movement has occurred. The instru- 
ment is also available with a graduated 
scale that measures the amount of move- 
ment numerically. 

This instrument can be used where trav- 
el must not be impeded and over permanent 
installations, such as belt lines, haul- 
ageways, etc., where other instruments 
are impractical. 

Availability 

The Guardian Angel is available from 
Conkle, Inc. The price is $30 per unit. 

Description 

The Guardian Angel consists of anchor 
clips, threaded rod, reference head, 
trigger mechanism, and reflector flag 
(fig. 8). 

Installation 

Step 1: Drill a 1-in-diam roof bolt 
hole to a stable zone of rock and deeper 
than the anchor horizon of the roof bolts 
in the test area. 

Step 2: Connect the desired lengths of 
rods, making sure to tighten the connect- 
ing locknuts. 




Scale, in 




FIGURE 8. - Guardian Angel. 

Step 3: Connect two anchor clips to 
the rod, making sure to tighten the lock- 
ing nuts. 

Step 4: Install the rods by manually 
pushing them upwards into the hole. 

Step 5: Slide the monitor assembly on- 
to the rod, then screw on the adjusting 
wingnut until the assembly is in contact 
with the roof (fig. 9). 

Step 6: To set the monitor for the de- 
sired amount of movement detection, latch 
the flag in the "up" position (as shown 
in figure 9). Turn in the adjusting nut 
until the flag drops. Back the adjusting 
nut off a few turns, relatch the flag, 
then turn the adjusting nut back in until 
it just unlatches the flag. Reset the 
flag by slightly backing off the adjust- 
ing nut. Using the adjusting nut wings 
for reference, back the nut off to the 
desired amount of movement to be detected 
( 3) : Each quarter turn equals 0.015 in, 
etc. 




Scale, in 

FIGURE 9. - Cutaway view of roof and in- 
stalled Guardian Angel (2). 



Data Collection 

This instrument is designed to give a 
visual indication of roof movement. The 
flag drops when the preset amount of 
movement has occurred. The amount of 
movement can be determined by either of 
the following methods (2). 

Method 1: If it is desirable to know 
the amount of roof sag prior to the 
flag's dropping, screw in the adjusting 
nut while counting the number of turns 
until the flag drops. Subtract the inch- 
es of deflection corresponding to the 
number of turns from the preset inches of 
deflection to obtain the amount of move- 
ment that has occurred. The system must 
now be reset to the previous, or a new, 
preset amount of movement to be detected 
(step 6 of installation). 

Method 2: If the flag has dropped, 
back off the adjusting nut while counting 
the turns until the flag just resets. 
Screw the nut in while counting the turns 
until the flag unlatches. Subtract the 
second count from the first, and add the 
corresponding inches of deflection to the 
original preset amount of movement to ob- 
tain the total amount of movement that 
has occurred. Again, the instrument must 
be reset if additional movement detection 
is desired. 

Each instrument can be observed as 
often as desired. For short-term instal- 
lations or potential movement areas, 
observations can be made each shift, 
daily, or weekly. For long-term, stable 
areas , observations can be made weekly or 
monthly. 

Data Interpretation 

The Guardian Angel detects a preset 
amount of roof sag. The preset amount is 
arbitrary. Allowable movement limits 
must be determined for each mine, based 
on prior experience and/or ongoing in- 
strumentation. Values for safe movement, 
movement indicating a need for caution, 
and movement indicating unstable roof 
must be established. The process of 
determining these values can be time con- 
suming and can require hundreds of mea- 
surements. From these values, the appro- 
priate presetting for movement detection 



can be determined. The values, however, 
may not be constant throughout the mine 
area. 

Horizontal Roof Strain Indicator 

Principle and Application 

The horizontal roof strain indicator 
(HORSI) is a device that measures the 
horizontal roof strain that accompanies 
roof sag preceding a roof fall (4). It 
is based on the principle that the hori- 
zontal distance between two roof bolts 
will increase as the roof sags, producing 
a measurable strain difference that can 
indicate stable or unstable roof condi- 
tions (fig. 10). 

Availability 

The HORSI is not yet commercially 
available. Consequently, it must be fab- 
ricated in-house. Cost of fabrication 
and materials is approximately $50; an 
additional $35 to $50 is needed to pur- 
chase the dial gauge. 



Roof bolts - 



;//;;;;//;;/////// \j; ;/;?;??;; s- y . '■'■'■' •'.'.'. '■ ' . , . ■.'.'.' /.■/.' 




I 




STATIFIED ROOF BEFORE SAG 



,'{'.'/{ i '.i '.'. '77 



Compression 



/ / 1 '.'. j {j ■ 




^m^^^^^^j^^^^m^,. 



no " 



STATIFIED ROOF AFTER SAG 



KEY 
L Original distance between bolts 
a l Distance change of left bolt 
£p Distance change of right bolt 



Horizontal strain ■ 



^L-t-^R 



FIGURE 10. - Cross section of a typical coal mine 
opening i llustrating the HORSI principle (4-6). 



10 



Description 



Data Collection 



The HORSI consists of two caps that 
attach to two adjacent boltheads with 
setscrews, a length of piano wire, a 
spring-tensioned plunger, a fitted col- 
lar, and a standard dial gauge indicator 
(fig. ID- 

Installation 

If installation is to be in the face 
area, the two bolts should be instrument- 
ed immediately after installation, be- 
cause the strain usually stabilizes with- 
in 2 days, unless the roof is trending to 
an unstable condition. In previously 
mined and bolted areas , the HORSI can be 
installed whenever desired (_5 ) . 

Step 1: Attach one of the bolt caps 
(anchor cap) to the head of one of the 
roof bolts, using the setscrews to secure 
it. 

Step 2: Attach the spring-loaded 
plunger cap (measuring cap) to the other 
roof bolt head, using the setscrews to 
secure it. The removable dial gauge is 
connected to this cap by a fitted collar. 

Step 3: Attach the piano wire to the 
spring plunger. Use a wire clamp to fas- 
ten it securely. 

Step 4: Connect the free end of the 
piano wire to the other bolt cap. The 
wire must be lightly tensioned, then se- 
cured with the setscrews. 

Step 5: Use the dial gauge indicator 
to take an initial reading, which will 
serve as the reference point for all ad- 
ditional readings. 



Roof bolts- 




fitted collar Spring-tensioned 
_,. , plunger 

Removable dial 

indicator 9 



Music 
wire 



Scale, In 



FIGURE 11. - Horizontal strain measuring 
apparatus (4-6). 



After the initial reading, the dial 
gauge should be read on each shift for 
the first 2 days after installation, 
then once a week if the readings have 
stabilized. If the readings have not 
stabilized, they should continue to be 
taken on each shift until stabilization 
or roof failure occurs. In old workings, 
which may be considered stabilized, read- 
ings can be taken daily or weekly, as de- 
sired. Any unusual changes should be re- 
ported to the appropriate mine personnel 
immediately. 

Data Interpretatio n 

Previous experiments by the instrument 
developers have shown that a 0.0001-in/in 
strain within 2 days could indicate un- 
stable roof conditions (4, 7-8). How- 
ever, this may not be the case in all 
mines. Mine operators must determine, 
through experiments and use of this in- 
strument, what amount of strain will in- 
dicate unstable roof conditions for a 
particular mine. 

Infrared Scanner 

Principle and Application 

The infrared scanner senses temperature 
differences between loose rocks and their 
background and expresses those differ- 
ences as a visual image (9) . Temperature 
differences appear as light and dark ar- 
eas on the visual image. The infrared 
scanner is one of two instruments that 
detect loose rock from a safe distance. 
(The other is the infrared thermometer.) 

Availability 

This instrument is still in the experi- 
mental stage with respect to use under- 
ground. The Bureau of Mines has a pro- 
totype that may soon be commercially 
available. Scanners approved by the U.S. 
Mine Safety and Health Administration may 
soon be ayailable from Hughes Aircraft 
Co.; Seco, Standard Equipment Co.; and 
Wahl Instruments, Inc. The approximate 



11 



cost of the infrared scanner ranges from 
$8,000 to $9,000. 

Description 

The infrared scanner is a handheld, 6- 
lb instrument, powered by a 6-V recharge- 
able battery (fig. 12). It produces a 
visual image of the heat waves from the 
material viewed. 



Installation 

No installation is needed, except to 
connect the battery to the scanner (some 
have internal batteries). 

Data Collection 

Turn the scanner on, point it toward 
the area to be observed, look through the 
eyepiece at the image, focus if neces- 
sary. No numerical data are obtained, 
only a visual picture of the viewed area, 
which cannot be recorded for future 
reference. 

The scanner should be used at the face 
prior to mining or roof bolting, at roof 
falls before cleanup begins, in air- 
courses where loose rock is likely to de- 
velop, and any place where roof can be 
tested only from a distance. 




Scale, in 



Data Interpretation 

The visual image will show light areas 
for warmer temperatures and dark areas 
for colder temperatures. Generally, 
loose rock will be colder and will pro- 
duce the darker image, but this does not 
hold true 100 pet of the time. Care must 
be taken to accurately determine which 
rocks are loose before attempting to cor- 
rect the hazardous conditions. Also, ex- 
ternal heat sources such as mining equip- 
ment may cause false readings. 

Infrared Thermometer 



Principle and Application 

Infrared thermometers are designed for 
temperature measurements where direct 
contact is impossible or impractical. 
They may be useful in mines to detect 
temperature differences between loose and 
stable roof rock; however, it has yet to 
be definitely established that they can 
perform this function. 

Availability 

Infrared thermometers are available 
from the following suppliers: Barnes 
Engineering Co.; Extech International 
Corp.; Industrial Products Co.; Mikron 
Instrument Co., Inc.; Raytek, Inc.; Seco, 
Standard Equipment Co.; and Wahl Instru- 
ments, Inc. The approximate cost ranges 
from $400 to $1,715. 

Description 

The infrared thermometer is handheld, 
with the gauge on the back facing the 
operator. It is shaped like a pistol 
with barrel and pistol grip. Infrared 
thermometers are made with different tem- 
perature ranges, either Fahrenheit or 
Celsius. Most are autocalibrating; some 
have sights for locating the object to be 
measured (fig. 13). 

Installation 



FIGURE 12. - Infrared scanner MO). 



No installation is necessary, but the 
thermometer must be calibrated, either 
manually or automatically. 



12 



SIDE VIEW 



3ACK VIEW 



>/ 2 ~in LCD display 




Ernissivity 



Connection for 
battery charger 



FIGURE 13. - Typical digital-readout infrared 
thermometer (Y\). 

Data Collection 

Point the instrument at the roof area 
to be measured. Distance from object 
should be 20 ft or less. This means that 
the area can be monitored while the per- 
son monitoring remains under supported 
roof. Slowly scan the area to detect 
any significant changes in temperature. 
Loose rock will be either warmer or cold- 
er, depending on whether the mine air is 
warmer or colder than the solid rock. 

There is no specific time span for data 
collection. Possible collection could be 
at the face before any mining operations 
begin. This instrument could also be 
used in airways, escapeways, etc., when 
the need to detect loose roof arises. It 
might also be used at roof falls before 
cleanup begins, to detect any remaining 
loose rock. 

Data Interpretation 

Loose rock will register either as 
warmer or colder. Temperature differ- 
ences will be minute, on the order of 
0.36° to 0.9° F (9). Such small dif- 
ferences may not be detectable by the 
present infrared thermometers, which are 
sensitive only to 1° F. External heat 



sources may also affect the accuracy of 
this instrument. 

Plumb Bob 

Principle and Application 

A plumb bob is a pointed weight that is 
suspended by a string. It can be used 
for imprecise measurement of entry con- 
vergence. This is accomplished by sus- 
pending the plumb bob from a point on the 
roof line and measuring its movement with 
respect to a reference pin in the floor 
(12). A plumb bob alone cannot dif- 
ferentiate between roof sag and floor 
heave. 

Availability 

Plumb bobs and associated materials are 
available from most hardware stores. The 
approximate cost ranges from $10 to $20. 

Description 

Instrumentation consists of a plumb 
bob, string, an eyebolt (which fastens to 
the roof bolt head) , and a reference 
pin that is grouted into the mine floor 
(fig. 14). 

Installation 

Step 1: Drill and tap the head of a 
roof bolt to accept the eyebolt. 

Step 2: Screw the eyebolt into the 
roof bolt and tighten. 

Step 3: Using the plumb bob as guide 
(hanging from the eyebolt), drill or chip 
a hole into the floor vertically below 
the eyebolt. 

Step 4: Grout the reference pin in the 
floor hole. This pin should be flat with 
a slight indentation at the center to 
accept the point of the plumb bob. 

Step 5: When the grout has hardened 
(15 to 30 min) , hang the plumb bob so 
that its point just touches the center of 
the floor reference pin. 



13 




n 



/ ^ 

&« Eyebolt 



-String 



Mine floor 



\ 



Not to scale 




Plumb bob 
Reference pin 



Grout 



FIGURE 14. - Plumb bob setup for roof con- 
vergence measurement. 

Data Collection 

When the plumb bob starts to lean, 
the entry has begun to converge. If 
numerical values are desired, the plumb 
bob should be hung a specified dis- 
tance above the floor reference pin. The 
amount of movement is the change in dis- 
tance from plumb bob to pin. 

A plumb bob installation should be 
checked as often as considered neces- 
sary. The frequency of observations 
should increase if substantial movement 
is detected. 



Data Interpretation 

The amount of movement indicating un- 
stable roof or floor conditions var- 
ies from mine to mine. A continuing 
ground control instrumentation program 
can define this movement. In addition, 
humidity will cause slight variations 
in string length, resulting in reduced 
accuracy. 

Spider Roof Monitor 

Principle and Application 

The Spider roof monitor is designed to 
signal roof movement (roof sag) (13). It 
attaches to an existing roof bolt in min- 
utes and requires no special tools or 
drilling. It gives a visual display when 
a specified amount of movement has oc- 
curred, and can be reset to measure addi- 
tional movement. 

Availability 

The Spider is available from The Spider 
Inc. The approximate cost is $30. 

Description 

The Spider consists of a plastic hous- 
ing, latching mechanism, pin, reflective 

canister, roof-contact actuating arms, 

and mounting screws (fig. 15). Roof 

movement moves the actuating arms , which 

release the latching mechanism, which 
lets the reflective canister drop. 

Installation 



sen the setscrew on the ac- 
and let the arms swing 

sen the bolthead-connecting 

tall the Spider on a roof 

secure by tightening the 

is recommended that the 

ailed on a bolt, minus the 

which is anchored 12 in 

ting bolts and extends 1 or 

roof line (fig. 16). 



Step 1: 


Loo 


tuating arms 


freely. 




Step 2: 


Loo 


setscrews . 




Step 3: 


Ins 


bolt head 


and 


setscrews . 


It 


Spider be 


inst 


bearing pi 


ate , 


above the 


exis 


2 in below 


the 



14 




SciSe, in 



FIGURE 15. - The Spicier roof monitor (13). 



12 in above 
existing bolts 






Remove existing 
bolt plate 




( < AY //.// ^<^/r<<< i \ /// y ,< V y// //A]r y U . 



«■*! 



m- 



Not to scale 



FIGURE 16. - Installation diagram for the 
Spicier roof monitor (13). 

Step 4: Set the actuating arms against 
the roof and secure them in posit* on by 
tightening the setscrews. The amount of 
movement detected will vary according to 
the location of the actuating arms (the 
installation pressure against the roof). 

Step 5: Latch the reflective drum by 
pushing it and the pin upward until they 
latch. If they will not latch, the actu- 
ating arms are applying too much pressure 
against the roof and must be readjusted. 

Data Collection 

If sufficient roof movement has oc- 
curred to trip the mechanism, the reflec- 
tive canister can be seen. This instru- 
ment can be viewed on a regular basis 
(weekly, monthly, etc.) or during regu- 
lar travel throughout the mine. If the 



instrument has tripped, it should be re- 
ported to the appropriate mine authority 
immediately. 

The detection of movement must be in- 
dividually analyzed for each mine, pos- 
sibly for each specific installation. 
The amount of movement that indicates 
potentially hazardous roof will vary from 
site to site. Basically, this instrument 
detects movement only; it does not re- 
veal the subsequent effect of the move- 
ment. Once the Spider has tripped, and 
the reflective canister is visible, it 
should be assumed that a hazardous roof 
condition has developed, until proven 
otherwise. 

SRC Closure Rate Instrument 

Principle and Application 

The SRC closure rate instrument pro- 
vides roof-to-floor closure rate measure- 
ments during underground mining, espe- 
cially retreat mining of coal pillars 
(14) . Change in resistance resulting 
from closure along a simple potentio- 
metric extensometer is relayed to a read- 
out box by means of a long cable. The 
instrument is designed to be pulled out 
of place and dragged to a safer location 
when closure reaches a predetermined 
rate. 

Availability 

This instrument can be purchased from 
Serata Geomechanics , Inc. Expected cost 
range is $1,000 to $2,000. 

Description 

The closure rate instrument system con- 
sists of a rugged telescoping potentio- 
metric extensometer and a digital readout 
and control box. The extensometer is de- 
signed to accommodate a height of from 
4.6 to 12.1 ft, with a measurement range 
of 6 in. It is spring loaded over this 
range. Long cables (98 to 125 ft) con- 
nect the extensometer to the readout box, 
permitting the operator to remain in a 
safe, supported area. A breakaway fea- 
ture on each extensometer allows it to be 
pulled from the fall area by its elec- 
trical cable (15). 



Installation 

The instrument is placed between the 
roof and floor in the desired measurement 
area, while the readout box is kept in a 
safe area. 



15 



mechanism, and two snaphooks (figs. 17- 
18). Anchoring stations are usually 
rods, grouted in place, with an eyebolt 
for connection to the instrument. 

Installation 



Data Collection 

The operator watches the digital read- 
out on the control box during pillar min- 
ing. When a predetermined, critical clo- 
sure rate is reached, an alarm light and 
horn are activated, and the operator re- 
trieves the extensometer and signals the 
miner operator to pull back. 

Data Interpretation 

Initially, the mine must determine a 
closure rate that indicates a roof fall 
is about to occur. This is the rate at 
which the alarms should be set to be ac- 
tivated. In many cases, a rate at which 
most falls will occur within 2 min of 
signalling can be determined. 

Tape Extensometer 

Principle and Application 



When measuring roof-to-floor conver- 
gence: 

Step 1: Drill and tap roof bolt head. 
Screw the eyebolt into the roof bolt 
head. 

Step 2: Directly below the roof sta- 
tion, drill a floor hole, and grout the 
floor station in place (eyebolt already 
attached) . 

When measuring rib-to-rib closure: 

Step 1: At the desired location, drill 
a horizontal hole into the rib, then 
grout the anchor station in place. 

Step 2: Repeat step 1 for the remain- 
ing rib anchor station. 

Rib and floor stations should be made 
from a minimum 12-in rod, which should be 
completely grouted into the hole to en- 
sure that the station is permanent. 



The tape ex 
convergence or 
the change in d 
manent stations 
(closure) or 
gence) . The s 
much as 100 ft 
tage this inst 
extensometer. 



tensometer detects entry 
rib failure by measuring 
istance between two per- 
, either from rib to rib 
roof to floor (conver- 
tations can be located as 
apart, which is an advan- 
rument has over the tube 



Tape extensometer housing 




p=M3> 



Steel tape 



FIGURE 17. - Side view of tape extensometer (16). 



Availability 

Tape extensometers are available from 
the following suppliers: Geokon, Inc.; 
Irad Gage; Roctest, Inc.; Sinco, Slope 
Indicator Co.; and Soiltest, Inc. The 
approximate cost ranges from $550 to 
$1,500. 

Description 

A tape extensometer consists of a 
steel engineers' tape, a dial-tensioning 



_ , .... . ... Tensioning /Thrust beoring 

Spring plunger Window with screw \ 

index marks 




Spring housing^ 

Compression^/ 

^ ' g Dial qougey 

housing - 

2 

i ■ 

Scale, in 



Rotating / 
shaft/ 



FIGURE 18. - Top view of tape extensometer (16). 



16 



Data Collection 



Tube Extensometer 



Readings are taken as follows (fig. 
19): 

Step 1: Connect the free end of the 
tape to one of the stations. 

Step 2: Connect extensometer (dial 
gauge) end to the remaining station. 
This should be the station at which the 
dial gauge is most easily read. 

Step 3: Tension the tape by turning 
the handle, then connect the extensometer 
body to the tape by inserting the hook 
into one of the holes in the tape. 

Step 4: Adjust the system to the 
zero mark on the extensometer spring 
plunger. 

Step 5: Measurement is obtained by 
adding the tape distance (where the hook 
is in the hole) to the reading on the 
dial gauge. 

Normally one reading per week is ade- 
quate, but large changes require more 
frequent readings, while small changes 
require less frequent readings. It 
should be noted that high air velocity 
will affect accuracy because of tape 
flutter (1_7). 

Data Interpretation 

The amount of measured movement indi- 
cating unstable conditions depends on the 
mine. Only through experimentation can 
this value be determined. 



Principle and Application 

A tube extensometer detects entry con- 
vergence (roof sag and/or floor heave) by 
measuring the change in distance between 
pairs of permanent stations anchored in 
the roof and floor of a mine (18). The 
readout system is either a dial gauge, a 
sonic probe, or a continuous drum. 

Availability 

Tube extensometers are available from 
the following suppliers: Geokon, Inc.; 
Irad Gage; Sinco, Slope Indicator Co.; 
and Soiltest, Inc. The approximate cost 
of a tube extensometer and readout system 
is from $690 to $3,000. 

Description 

A tube extensometer consists of a se- 
ries of telescoping tubes of Invar steel 
(19) (or other suitable metal alloy) , an 
internal spring that provides tension 
against the reference stations, two ref- 
erence stations, and a readout system 
(dial gauge, sonic probe, or continuous 
drum) (fig. 20). The sonic probe has an 
accompanying readout box and electrical 
connection cable. The continuous readout 
has a rotating-drum strip chart, and a 
lever arm and pen, which records all 
movements. 



Index marks not aligned 



Index marks aligned 




Scale, in 
FIGURE 19. - Index mark alignment for correct reading of tape extensometer (26). 



17 



o 



//xxv^/it Kkwy/xw 




"W//T 




Compression 
spring 



Dial indicator 
(2-in range) 




Not to scale 



^/avw/awv/ 



Installation 

To anchor the reference stations: 

Step 1: Drill and tap a roof bolt head 
to accept the roof reference pin. 

Step 2: Use a plumb bob to mark the 
floor directly below the bolt, then drill 
a hole 12 in deep into the floor. 

Step 3: Grout the floor reference sta- 
tion into the floor hole. 

Step 4: Screw the roof station into 
the roof bolt head and tighten securely. 

Step 5: When the grout has hardened, 
connect the appropriate number of tubes 
together, place the extensometer between 
the reference stations and record the 
first reading; this reading serves as the 
reference for all subsequent readings. 

Data Collection 

The collection procedure depends on the 
type of tube extensometer readout being 
used. For the continuous-recording type, 
all that is required is to observe the 
readings recorded on the strip chart and 
change the chart periodically. The dial 
gauge only needs to be read. The sonic 
probe readout requires connecting the 
readout box to the extensometer using the 
electrical cable, then reading the de- 
flection as displayed. Data should be 
collected once a week unless conditions 
warrant otherwise. 

Data Interpretation 

Interpretation of data depends on the 
mine in which the data are collected. 
Unacceptable movement must be deter- 
mined for each mine through continuing 
experimentation. 

Vi sual Roof Sag Bolt 

Principle and Application 



FIGURE 20. - Dial-gauge tube extensometer (20). 



As its name 
displays roof 



implies, the instrument 
movements visually, and 



18 



gives no numerical data. This is not a 
precise measuring instrument, but it is 
useful for detecting impending roof fail- 
ure, at minimal cost, in time to provide 
additional support (21) . Visual roof sag 
bolts are intended for installation in 
the face area but can be installed mine- 
wide. They do not replace pattern bolts 
since they have no supporting capability. 

Availability 

Visual roof sag bolts are not commer- 
cially available, but the materials to 
fabricate them are. Since a standard 
roof bolt is used, the only material to 
be purchased is the reflective tape or 
paint. Total cost will range from $5 to 
$15. 

Description 




FIGURE 21. - Visual roof sag bolt at installation (19). 



A visual roof sag bolt system consists 
of a standard mechanical anchor bolt, 
three bands of reflective tape or paint 
(green, yellow, and red), and a poly- 
styrene foam (such as Styrofoam) plug 
(fig. 21). The roof bolt can have the 
head left on or cut off and should be at 
least as long as (preferably longer than) 
the pattern bolts in the area being 
monitored. The movement-indicating bands 
(approximately 0.5 in wide) are placed on 
the bolt with the green nearest the an- 
chor, followed by the yellow and then the 
red, going toward the head of the bolt. 
A polystyrene foam plug, cut to slide 
over the bolt and into the hole, serves 
as the reference level indicator. 



Step 3: Replace the anchor, slide the 
bolt into the hole, and seat the foam 
plug. 

Step 4: Tighten the bolt so that all 
three color bands just show. 

Data Collection 



No numerical data are collected. A 
quick glance tells mine personnel if the 
roof has moved. The bolts can be checked 
either randomly in passing, or on a regu- 
lar schedule. The frequency of observa- 
tion depends on the amount of movement 
occurring. The appropriate mine person- 
nel must be notified of any significant 
movements as soon as possible. 



Installation 

The bolts should be installed in the 
middle of the entry or at intersections 
where, theoretically, the most movement 
will occur. 

Step 1: Drill a standard roof bolt 
hole the length of the visual roof sag 
bolt. The bolt should anchor above the 
anchorage horizon of the surrounding pat- 
tern bolts in a stable zone of rock. 

Step 2: Remove the anchor and slide 
the foam plug down over the bolt to the 
bolt head. 



Data Interpretation 

The amount of movement (indicated by 
the disappearance of the color bands) 
will vary from mine to mine. Each mine 
must determine the rate of movement that 
indicates unstable roof conditions. The 
color bands are interpreted as follows: 

1. All three colors showing: No move- 
ment, stable roof conditions (fig. 21). 

2. Green disappearing: Initial (slight) 
movement, caution, possible unstable roof 
conditions developing (fig. 22.4). 



19 




Mine roof 



__ 



Foam plugs 




Scale, in 



B 



© 



FIGURE 22. - Visual roof sag bolt after convergence movements. A, Initial (slight); B, moderate; 
C, substantial. 



3. Yellow disappearing: Moderate move- 
ment, caution, unstable roof conditions 
developing (fig. 225) . 

4. Red disappearing: Substantial move- 
ment, warning, unstable roof conditions 
(fig. 22C). 

REFERENCES 

1. Glowlarra (White Pine, MI.). Rock 
Fall Warning Devices. Brochure, 1979, 
1 p. 

2. Conkle Inc. (Paonia, CO.). The 
Guardian Angel. Brochure, 1979, 2 pp. 

3. Guccione, E. Conkle' s Warning Mon- 
itor: The Miners' Guardian Angel. Coal 
Min. & Process., v. 15, No. 9, 1978, 
pp. 106-108. 

4. Chironis, N. P. (ed.). Homemade 
Roof-Strain Indicator Helps Judge Safety 
of Bolted Coal Mine Roof. Sec. in Coal 
Age Operating Handbook of Underground 
Mining. McGraw-Hill, v. 1, 1977, p. 218. 

5. U.S. Bureau of Mines. Horizontal 
Roof Strain Indicator (HORSI). Technol- 
ogy News, No. 2, 1974, 2 pp. 

6. Panek, L. A. Evaluation of Roof 
Stability From Measurements of Horizontal 
Roof Strain. Paper in Ground Control As- 
pects of Coal Mine Design. Proceedings: 
Bureau of Mines Technology Transfer 



Seminar; Lexington, Ky.; March 6, 197 3, 
comp. by Staff, Mining Research. BuMines 
IC 8630, 1974, pp. 92-96. 

7. McDowell, C. D. Coal Safety. 
Min. Eng. (N.Y.), v. 25, No. 2, 1973, 
p. 97. 

8. Radcliffe, D. E. , and R. M. State- 
ham. Effects of Time Between Exposure 
and Support on Mine Roof Stability, Bear 
Coal Mine, Somerset, Colo. BuMines RI 
8298, 1978, 13 pp. 

9. Chironis, N. P. (ed.). New Infra- 
red Scanner Helps Spot Hazardous Condi- 
tions in Mines. Sec. in Coal Age Oper- 
ating Handbook of Underground Mining. 
McGraw-Hill, v. 1, 197 7, pp. 212-216. 

10. Hughes Aircraft Co. (Carlsbad, 
CA) . Probeye Infrared Viewers. Bull. SL 
2491, 1980, 4 pp. 

11. Mikron Instrument Co., Inc. 
(Ridgewood, NJ) . Digital Infrared Ther- 
mometers. Brochure M80-879-10M, 1980, 
8 pp. 

12. Shepherd, R. , and D. P. Ashwin. 
Measurement and Interpretation of Strata 
Behavior on Mechanized Faces. Colliery 
Guardian, v. 216, No. 12, 1968, pp. 795- 
800. 

13. The Spider Inc. (St. Louis, MO). 
The Spider. Brochure, 1980, 1 p. 



20 



14. U.S. Bureau of Mines. Roof to 
Floor Closure Rate Instrument for Under- 
ground Mines. Technology News, No. 136, 
1982, 2 pp. 

15. McVey, J. R. , and W. L. Howie. 
New Closure Rate Instrument for Retreat 
Mining Operations. Min. Eng. (N.Y.), v. 
33, No. 12, 1981, pp. 1699-1700. 

16. Terrametrics (Golden, CO). In- 
struction Manual, Tape Extensometer. Un- 
dated, 6 pp. 

17. Mann, C. D. , and J. J. Reed. St. 
Joe Builds Practical Rock Mechanics 
Tools. Eng. and Min. J., v. 162, No. 3, 
1961, pp. 100-106. 



18. Wang, C. Survey of Tools and 
Techniques for Roof Control Studies in 
Underground Coal Mines. BuMines, PMSRC 
Interim Report, Mar. 1972, 43 pp.; avail- 
able upon request from E. R. Bauer, Bu- 
reau of Mines, Pittsburgh, PA. 

19. Parker, J. How Convergence Mea- 
surements Can Save Money. Eng. and Min. 
J., v. 174, No. 8, 1973, pp. 92-97. 

20. Bauer, E. R. , and G. J. Chekan. 
Convergence Measurements for Squeeze Mon- 
itoring: Instrumentation and Results. 
BuMines TPR 113, 1981, 9 pp. 

21. Barry, A. J., and J. A. McCormick. 
Spotlight on Roof Control. Coal Min. & 
Process., v. 3, No. 2, 1966, pp. 21-22. 



CHAPTER 2.— STRATA SEPARATION MEASUREMENTS 



INTRODUCTION 

Strata separation is the differential 
downward separation of distinct roof 
strata layers. It results from a low co- 
efficient of friction between roof strata 
layers and the weight of the immediate 
roof and overburden. 

Strata separation measurements allow 
mine operators to detect movement be- 
fore roof control problems occur, to 
analyze and prevent further occurrences, 
and to make changes in mine plans. 
Early detection of developing hazardous 
roof conditions can improve safety and 
production. 

GROUND MOVEMENT INDICATORS 

Each ground movement is characterized 
by specific indicators. However, since 
some indicators are common to several 
types of movement, recognizing the move- 
ment that is occurring can be difficult, 
and at times may be just an educated 
estimate. The following indicators of 
strata separation can help to identify 
this movement: 

• Cracks developing in middle of roof 

span. 

• Distinct layers of roof falling. 

• Succession of falls of distinct 

layers in the same area. 

• Increase in dripping of water from 

roof. 

• Visible entry closure or convergence. 



INSTRUMENT SELECTION 

As requirements and parameters are de- 
fined, they should be listed on an in- 
strument selection worksheet (fig. 1). 
In this case, the user should list strata 
separation in the "movement occurring" 
column. The controlling parameters, as 
defined in the section "Instrument Selec- 
tion Guidelines," should be listed in the 
"desired elements" column. 

Next, the user should refer to figures 
23 through 25 and follow steps 1 through 
6 to make a preliminary choice as to 
which instrument(s) best suits the ground 
control problem being investigated. 

Step 1: List the instruments available 
for monitoring strata separation on the 
worksheet under "instruments available" 
across from "movement occurring. " All 
three instruments described in this chap- 
ter will monitor strata separation. 

Step 2: From figure 23, choose the 
instruments that satisfy the cost param- 
eter selected and list them under "in- 
struments available," across from the 
cost parameter. 

Step 3: From figure 24, choose the in- 
struments that satisfy the technical as- 
pects parameter and list them under "in- 
struments available," across from the 
technical aspects parameter. 

Step 4: From figure 25, choose the in- 
struments that satisfy the data require- 
ment parameter and list them under "in- 
struments available," across from the 
data requirement parameter. 



21 



Step 5: Examine the "instruments 
available" column of the worksheet to 
find the instruments that satisfy all re- 
quirements and parameters, and list them 
at the bottom of the worksheet. If no 
instruments satisfy all the requirements 
and parameters, it may be necessary to 
change the parameters or to choose the 



instrument that satisfies the most re- 
quirements and parameters. 

Step 6: At this point, the user must 
make a decision as to the instrument or 
instruments to be used, based on the fol- 
lowing detailed descriptions. 

DESCRIPTION OF INDIVIDUAL INSTRUMENTS 



Instrument 


Cost of purchase or fabrication 


Dollars 

50 100 250 500 IJOOO 2,000 10,000 

i . , , i . i . . i , . i , . . r i , i i 


Borehole 
extensometer 




i 




Simple weighted 
bed seporotion 
indicator 


a 


Strotoscope 




! 1 





KEY 



3 Cost range 



FIGURE 23. • Cost range of purchase or fabrica- 
tion of strata separation measuring instruments. 



Instrument 



Borehole 
extensometer 



Simple weighted bed 
separation indicotor 



Range of technical ability required 



Slight 



i^WW^ 



Moderate Extensive 



\WV\>^W\VJ 



T' 5SS . 3 



Strotoscope 



UK AUUv,v^ 



KEY 
Installation 
Monitoring 



13 Dato interpretation 



FIGURE 24. - Range of technical ability required 
for installation, monitoring, and data interpretation 
of strata separation measuring instruments. 



Instrument 


Type of measured data obtained 


Visual 


Simple numerical 


Detailed numerical 


j Borehole 

-IOmeter 




o 


o 


l-rtt *e -.'•--. bed 




o 




Strotoscope 





o 





<| f 

Q Data obtainable 

FIGURE 25. - Type of measured data obtained 
from strata separation measuring instruments. 



Borehole Extensometer 

Principle and Application 

Borehole extensometers are in-hole mea- 
suring devices that detect strata separa- 
tion movements at various horizons of the 
roof. Movement is detected by the change 
in distance between anchors at vari- 
ous depths and the reference head at the 
roof line ( 1_) . 6 Anchors are connected 
to the reference head by rod or wire 
connection systems. Borehole extensom- 
eters are available as single-position 
(one horizon detection) (fig. 26A) and 

"Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this chapter. 




-Deep C-anchor 



-Connecting rod 



Scale, in 



■Roof level anchor 



-End cap 
-Deep C-anchor 

Ring magnets 
-Anchor tube 



Scale, in 



-Magnet 

■Roof level shel 
anchor 



FIGURE 26. - Borehole extensometer. A, Single 
point; B, multipoint (2). 



22 



multiple-position (multiple horizon de- 
tection) instruments (fig. 26B) . Read- 
out systems include flexible and rigid 
sonic probes, dial gauge, and spring 
cantilever. 

Availability 

Borehole extensometers can be purchased 
from the following suppliers: Geokon, 
Inc.; Irad Gage; Roctest, Inc.; Sinco, 
Slope Indicator Co.; and Soiltest, Inc. 

Approximate cost for the extensometer 
and readout system ranges from $100 to 
$1,800. 

Description 

Borehole extensometers consist of an- 
chors, a reference head, wire or rod con- 
necting assemblies, and a readout system. 
Anchors are of many different types: 
rock bolt expansion shell, double-wedge 
expansion shell, flat sliding wedge, 
wedge-split ring, screw-activated shoe, 
hydraulic anchor, cement-grouted anchor, 
C-anchor, cam anchor, and expansion shoe 
anchor. The readout system can be a 
dial gauge, depth gauge, dial microm- 
eter, continuous-drive mechanical chart, 
spring cantilever, potentiometer, or son- 
ic probe. 

Installation 

Installation procedures are very gen- 
eral, because each type of anchor has a 
specific installation sequence that can- 
not be adequately described here. In- 
formation on special tools or methods 
needed to install the connecting rods or 
wires can be obtained from the instrument 
suppliers. 

Step 1: Drill anchor holes to appro- 
priate diameter and depth (just past the 
last horizon of desired monitoring). 

Step 2: Set anchors in the hole at de- 
sired horizons. 

Step 3: Install reference head in the 
hole at the roof line. 

Step 4: Connect anchors to reference 
head with wires or rods (depending on the 
type of extensometer being installed). 



Step 5: Connect the readout system to 
reference head and take the initial read- 
ings, which will serve as the baseline 
for all subsequent readings. 

Data Collection 

Data are collected using the readout 
system appropriate to the extensometer 
selected. 

A total of eight horizons in 15 ft of 
strata can be monitored for their change 
of location, to determine bed separation. 
Borehole extensometers should be read 
once a week unless (1) no movement is oc- 
curring (readings can be stopped) or (2) 
large movements are occurring (frequency 
of readings should increase) . 

Data Interpretation 

The critical factor is the amount of 
bed separation at each horizon monitored 
and the total roof sag created. The 
strata will sag differently from mine to 
mine; therefore, there is no degree of 
deflection that can be taken as an 
industry-wide warning of developing un- 
stable roof conditions. Each mine must 
determine allowable separation for the 
roof strata being supported. 

Simple Weighted Bed 
Separation Indicator 

Principle and Application 

The simple weighted bed separation 
indicator (also known as a vertical dis- 
placement gauge) can be adapted to mea- 
sure one or several horizons of separa- 
tion within the same hole. Movement is 
measured by the change in length of wires 
with reference to a brass plug at the 
roof line. 

Availability 

This instrument is not commercially 
available. It can be fabricated in-house 
or by a local machine shop. Cost for 
each installation is approximately $5 to 
$15. 



23 



Description 

Components include spring clip an- 
chors, lightweight steel wires, ring- 
tongue solderless terminals, a brass ref- 
erence plug, and a weight. Measurements 
are made with a vernier caliper or gradu- 
ated scale , depending on the precision 
required. 

Installation 



Step 1: Braze wires to anchors before 
installation, attaching the wires to cen- 
ter or sides of anchors as needed. 

Step 2: Drill 3-in-diam hole into roof 
to a depth greater than the zones of sus- 
pected separation. 

Step 3: Install anchors one at a time, 
making sure not to tangle the wires. To 
do this , compress the anchor in a hollow 
cylinder, position it at the desired 
depth, then eject it from the cylinder by 
pushing a rod through the cylinder. 

Step 4: Slide wires through appropri- 
ate holes in the brass plug, then secure 
the plug in the hole opening. 

Step 5: Clip wires and install ring- 
tongue solderless terminals (fig. 27), to 
which the weight will be attached. 

Step 6: Attach the weight to one wire 
at a time and take an initial reading to 
serve as the reference for all subsequent 
readings. 

Data Collection 

Readings are taken by hanging the 
weight on the wire, then measuring the 
distance from the solderless terminal to 
the reference plug. A vernier caliper 
works well, but a graduated scale will 
also work. A change in wire length indi- 
cates a separation of strata. 

Data Interpretation 

How much each zone has moved is shown 
by the amount of movement recorded in 
comparison with the reference reading. 
Subtracting the movement measured from 
the reference reading will give the total 
movement from the time of installation. 



Spring clip anchor 



Measuring 
wires — 



Scale, in 



Spring clip anchor 




6 



-Spring clip anchor 



6 



Movement indicated by 
| change in length 



®*. — Weight attached here 



FIGURE 27. - Simple weighted bed separation indicator. 

Interpretation of the data will depend 
on the mine or area being monitored. The 
amount of movement that indicates unsta- 
ble roof conditions must be determined 
for each mine. 

Stratascope 

Principle and Application 

The stratascope (borescope) is an opti- 
cal viewing instrument designed to permit 
visual or photographic observations with- 
in a drilled hole. It is used to detect 
cracks, separations, geologic makeup of 
mine strata, and lateral roof movements. 
Recently developed models use flexible 
f iberoptics. 



24 



Availability 

Stratascopes can be purchased from 
the following suppliers: American Op- 
tical Corp.; Baltimore Instrument Co., 
Inc.; Eder Instrument Co., Inc.; Expand- 
ed Optics Co., Inc.; Instrument Technol- 
ogy, Inc.; Lenox Instrument Co., Inc.; 
Olympus Corp of America; Soiltest, Inc.; 
and Welch Allyn, Inc. Approximate cost 
ranges from $390 to $10,000. 

Description 

The stratascope is basically a peri- 
scope, either handheld or tripod mount- 
ed. It consists of a light, a protec- 
tive outer housing, a lense, and mirrors 
or fiberoptics (fig. 28). The light 
is powered by a battery pack. If pic- 
tures are to be taken, tripods are need- 
ed for both the stratascope and camera 
(fig. 29). 




FIGURE 28. - Fiberoptic stratascope, battery 
pack, camera, and attachment (3). 



Installation 



Step 1: Drill a hole to the desired 
depth (the diameter depends on the 
stratascope used). 

Step 2: Assemble necessary extensions 
for viewing at the desired depth. 

Step 3: Connect the battery pack to 
the stratascope using electrical cable. 

Step 4: If photographs are to be ta- 
ken, assemble the stratascope tripod 
directly below the hole. 

Data Collection 

Visual observation: Slowly slide the 
stratascope into the hole, stopping to 
view the hole where desired, by looking 
through the eyepiece. By rotating the 
stratascope, the entire circumference of 
the hole can be observed. 

Photographic observation: Slide the 
stratascope into the hole, then clamp it 
to the tripod. Move the tripod-strata- 
scope assembly until the stratascope is 
centered in the hole. Set up the camera 
tripod and attach the camera to tripod 
and to stratascope. Set the stratascope 
to the borehole area to be photographed 
and take the picture. 




FIGURE 29. - Stratascope setup for roof observation. 



25 



There are no specific guidelines as to 
how often a hole should be viewed. Using 
the stratascope in newly mined areas to 
determine if the roof strata characteris- 
tics have changed or in previously mined 
areas to check for developing cracks and 
separations will determine the frequency 
of viewing. 

Data Interpretation 

Since no two mines are the same, the 
significance of any irregularities ob- 
served will depend on the particular 
mine. Irregularities such as clay veins, 
mud seams, cracks, fractured zones, and 
slips will create different problems 
for each mine. The important fact is 
that early detection of these ground 



conditions will enable mine personnel to 
initiate the appropriate action to con- 
trol them. 

REFERENCES 

1. International Society for Rock Me- 
chanics. Suggested Methods for Monitor- 
ing Rock Movements Using Borehole Exten- 
someters. Int. J. Rock Mech. and Min. 
Sci. and Geomech. Abstr. , v. 15, No. 6, 
1978, pp. 307-317. 

2. Irad Gage (Lebanon, NH). Geotech- 
nical Instrumentation. Catalog, 1980, 
40 pp. 

3. FitzSimmons, J. R. , R. M. Stateham, 
and D. E. Radcliffe. Flexible, Fiber- 
optic Stratascope for Mining Applica- 
tions. BuMines RI 8345, 1979, 12 pp. 



CHAPTER 3.— LATERAL ROOF MOVEMENT MEASUREMENTS 



INTRODUCTION 

Lateral roof movement is the differen- 
tial horizontal displacement (sliding) of 
distinct roof layers. It is a result of 
a low coefficient of friction between 
roof layers and a high horizontal stress 
within the roof. 

Lateral roof movement measurements pro- 
vide mine operators with a means of de- 
tecting such movements before roof con- 
trol problems occur, analyzing their 
cause and preventing further occurrences, 
and making necessary changes in mine de- 
sign. Early detection of developing haz- 
ardous roof conditions can result in in- 
creased safety and production. 

GROUND MOVEMENT INDICATORS 

Like other ground movements, lateral 
roof movement has specific indicators 
that characterize it. Unfortunately, 
some indicators are common to sever- 
al movements. Therefore, differentiating 
among movements and determining which 
movement is occurring can be difficult, 
and at times may be just an educated 
estimate. The following indicators of 
lateral roof movement should help in rec- 
ognizing its occurrence: 

• Offsets developing in holes drilled 
in the roof. 



• Falls where many bolts are bent in 

in the same direction. 

• Bolts shearing and falling out of 

holes. 

• Tension cracks in roof at one rib 

line, compression cracks in roof at 
opposite rib line. 

INSTRUMENT SELECTION 

As requirements and parameters are 
defined, they should be listed on an 
instrument selection worksheet (fig. 1). 
Since this chapter deals with lateral 
roof movement only, "lateral roof move- 
ment" should be written in the "move- 
ment occurring" column. The controlling 
parameters as defined in the section "In- 
strument Selection Guidelines," should be 
listed in the "desired elements" column. 

Next, the user should refer to fig- 
ures 30 through 32 and follow steps 1 
through 6 to make a preliminary choice 
as to the most suitable instrument(s) 
for the ground control problem being 
investigated. 

Step 1: Both of the instruments in 
this chapter satisfy the "movement occur- 
ring" requirement previously entered on 
the worksheet. List them under "instru- 
ments available," across from the "move- 
ment occurring" parameter. 



26 



Step 2: From figure 30, choose the in- 
strument that satisfies the cost parame- 
ter selected and list it under "instru- 
ments available," across from the cost 
parameter. 

Step 3: From figure 31, choose the 
instrument that satisfies the technical 
aspects selected and list it under "in- 
struments available," across from the 
technical aspects parameter. 

Step 4: Figure 32 shows that both in- 
struments provide visual and simple nu- 
merical data. Neither provides detailed 



Instrument 


Cost of purchase or fabrication 


Dollars 

10 50 100 250 500 1,000 2,000 10,000 

1 , , , 1 , 1 , , 1 , , 1 , . . , 1 , 1 1 


Plumb bob 




1 1 




Stratascope 




1 1 





KEY 



3 Cost range 



FIGURE 30. - Cost range of purchase or fabrica- 
tion of lateral roof movement measuring instruments. 



Instrument 


Range of technical ability required 


Slight 


Moderate 


Extensive 


Plumb bob 










^^^^^W 








Stratascope 










skkkkkkkkkkkkkkwww 





KEY 



1 Installation 
I Monitoring 



K\\\\l Data interpretation 
FIGURE 31. - Range of technical ability required 
for installation, monitoring, and data interpretation 
of lateral roof movement measuring instruments. 



Instrument 


Type of measured data obtained 


Visual 


Simple numerical 


Detailed numerical 


Plumb bob 


o 


o 




Stratascope 


o 


o 





KEY 

O Da 'a obtainable 

FIGURE 32. - Type of measured data obtained 
from lateral roof movement measuring instruments. 



data. List both or neither on the work- 
sheet depending on the type of data 
required. 

Step 5: From the "instruments availa- 
ble" column, determine which instrument 
satisfies all requirements and parame- 
ters. Enter this instrument at the bot- 
tom of the worksheet. If neither instru- 
ment satisfies all the requirements and 
parameters, it may be necessary to change 
the parameters or to choose the instru- 
ment that satisfies the most requirements 
and parameters. 

Step 6: At this point, the user must 
decide which instrument should be se- 
lected, based on the following detailed 
descriptions. 

DESCRIPTION OF INDIVIDUAL INSTRUMENTS 

Plumb Bob 

Principle and Application 

A plumb bob is a pointed weight sus- 
pended by a string. It can be used for 
imprecise measurement of lateral roof 
movement, by being suspended from a point 
on or in the roof and its movement mea- 
sured with respect to a reference pin in 
the floor. ' Observed plumb bob move- 
ments, coupled with visual observation of 
the holes drilled into the roof will be 
sufficient for detection of lateral move- 
ment of roof strata. 

Availability 

Plumb bobs and associated materials are 
available from most hardware stores. The 
approximate cost ranges from $10 to $20. 

Description 

The assembly consists of a plumb bob, 
string, a spring clip anchor, and a ref- 
erence pin grouted into the mine floor 
(fig. 33). 

'Shepherd, R., and D. P. Ashwin. Mea- 
surement and Interpretation of Strata Be- 
havior on Mechanized Faces. Colliery 
Guardian, v. 216, No. 12, 1968, pp. 795. 



27 



Suspected glide zone- 
Mine roof 



-Spring clip anchor 



Mine roof 



-Wire 



K- 



Entry 



-• — String 



= :] 



Ring-tongue solderless 
terminal 



FRib 



■- — Plumb bob 

-Reference pin 



Mine floor 
Not to scale 



12 



-Grout 



Mine floor 



FIGURE 33. - Plumb bob setup for detecting 
lateral roof movement. 



Installation 

Step 1: Braze a long piece of steel 
wire (longer than depth of hole) to the 
center of a spring clip anchor. 

Step 2: Drill a minimum 2-in-diam 
hole into the roof strata layer that is 
thought to be moving. 

Step 3: Install the spring clip anchor 
in the hole by compressing the anchor in 
a hollow cylinder, pushing this cylinder 
and anchor to the desired location, and 
then pushing the anchor out of the cylin- 
der and against the strata using a rod 
slid up through the cylinder. 



Step 4: Remove the cylinder and rod 
from the hole. 

Step 5: Clip the wire several inches 
below the hole, then clamp a ring-tongue 
solderless terminal to the end of the 
wire. 

Step 6: Using the suspended plumb bob 
as a guide, drill or chip a hole into the 
mine floor directly below the plumb bob. 

Step 7: Grout the reference pin in the 
floor so that its point is directly under 
the plumb bob point. 

Step 8: When the grout has hardened, 
adjust the plumb bob so that it is just 
barely above the reference pin. 

Data Collection 

Any roof movement will be indicated by 
a change in horizontal distance between 
the reference pin and the plumb bob. 

A plumb bob installation should be 
checked as often as experience indicates 
is necessary. The frequency of obser- 
vations should increase if substantial 
movement is detected. 

Data Interpretation 

The amount of movement that indicates 
unstable roof conditions will vary from 
mine to mine. Only by experience and 
continuing measurements can this movement 
be determined. Once this is known, de- 
veloping unstable roof can be detected in 
time for corrective action. 

Stratascope 

See chapter 2 for a complete descrip- 
tion of this instrument. 



CHAPTER 4. —STRESS MEASUREMENTS 



INTRODUCTION 



Ground stress is 
area distributed 
There are two type 
in situ (produced 
tion (resulting f 
stress measurement 
design. Based on 
stress around an 
orientation of ent 



the force per unit 
about an excavation, 
s of stress, either 
by nature) or extrac- 
rom mining). Ground 
s are used for mine 
the distribution of 
excavation, the best 
ries and the optimum 



pillar size can be determined, thus re- 
ducing or eliminating ground control 
problems. 

Stress measurements provide mine opera- 
tors with a means of detecting ground 
(roof, rib, and floor) control problems 
before and after they occur, analyzing 
their causes and preventing further oc- 
currences, and making necessary changes 
in mine design. 



28 



GROUND MOVEMENT INDICATORS 

Like other ground movements, ground 
stress has specific indicators that char- 
acterize it. Unfortunately, some indi- 
cators are common to several movements. 
Therefore, differentiating among sever- 
al movements can be difficult, and at 
times may be just an educated estimate. 
The following indicators of excessive 
stress can help miners to recognize its 
occurrence: 

• Excessive number of roof falls. 

• Numerous roof falls with same 

orientation. 

• Excessive rib sloughing. 

• Floor heave. 

• Roof sag. 

• Entry closure (squeeze). 

INSTRUMENT SELECTION 

As requirements and parameters are de- 
fined they should be listed on an instru- 
ment selection worksheet (fig. 1). Since 
this chapter deals with stress measure- 
ments only, stress should be listed under 
the "measurement desired" column. The 
controlling parameters, as defined in the 
section "Instrument Selection Guide- 
lines," should be listed in the "desired 
elements" column. 

Next, the user should refer to fig- 
ures 34 through 36 and follow steps 1 
through 6 to make a preliminary choice 
as to the most suitable instrument(s) 
for the ground control problem being 
investigated. 

Step 1: All of the instruments in this 
chapter satisfy the "measurement desired" 
requirement (stress) previously listed on 
the worksheet. List these instruments 
under "instruments available," across 
from the "movement occurring" parameter. 

Step 2: From figure 34, choose the in- 
struments that satisfy the cost parameter 
previously selected and list them under 
"instruments available," across from the 
cost parameter. 

Step 3: From figure 35, choose the 
instruments that satisfy the technical 
aspects parameter and list them under 



Instrument 


Cost of purchase or fabrication 


Dollars 

10 50 100 250 500 1,000 
1 .'. . 1 . 1 , . 1 . . 1 . . . . 1 


2,000 10,000 
1 ' 1 


Borehole defor- 
mation gauge 




1 1 




Borehole inclusion 
stressmeter 




1 1 




Borehole- mount 
strain gauge 




1 1 




CSIR strain 
gauge strain cell 




1 1 




CSIR triaxial 
strain cell 


a 


CSIRO hollow 
inclusion stress cell 




i i 




Cylindrical borehole 
pressure cell 


a 


Flat borehole 
pressure cell 


□ 


Flatjack 






i i 




Mechanical 
strain gauge 


D 


Surface-mount 
photoelasticgauge 






i i 




Surface-mount 
strain gauge 




i l 




Surface rosette 
undercoring 


a 


Vibrating wire 
stressmeter 




i i 





KEY 

in Cost range 



FIGURE 34. - Cost range of purchase or fabri- 
cation of stress measuring instruments. 

"instruments available," across from the 
technical aspects parameter. 

Step 4: From figure 36, choose the in- 
struments that satisfy the data require- 
ment parameter and list them under "in- 
struments available," across from the 
data requirement parameter. 

Step 5: From the "instruments availa- 
ble" column, determine the instruments 
that satisfy all requirements and parame- 
ters. List these at the bottom of the 
worksheet. If no instruments satisfy all 
of the requirements and parameters, it 
may be necessary to change the parameters 
or to choose the instruments that satisfy 
the most requirements and parameters. 

Step 6: At this point, the user must 
select the most suitable instrument(s) . 
The final decision can be made from the 
following detailed descriptions. 



29 




k\\\\1 Data interpretation 

FIGURE 35. - Range of technical ability required 
for installation, monitoring, and data interpretation 
of stress measuring instruments. 

DESCRIPTION OF INDIVIDUAL INSTRUMENTS 

Borehole Deformation Gauge 

Principle and Application 

Borehole deformation gauges are de- 
signed to measure diametral deformations 
of a borehole during the overcoring 
process of stress relief (l). 8 These de- 
formation measurements provide informa- 
tion to calculate the state of stress 
in the plane normal to the borehole. 
This technique determines the absolute 
field stress and is not easy to use, 
especially in coal measure rocks. The 
gauges are either single component (one 

"Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this chapter. 



Instrument 


~>pe of measured data oDtained 


Visual 


Simple numerical 


Detailed numerical 


Borehole 
deformation gouge 






o 


Borehole inclusion 
stressmeter 




o 




Borehole-mount 
strain gauge 






o 


CS 1 R strain gauge 
strain cell 






o 


CSI R tnaxial 
stram cell 






o 


CSIRO hollow 
inclusion stress cell 






o 


Cylindrical borehole 
pressure cell 






o 


Flat borehole 
pressure cell 




o 




Flatjack 




o 




Mechanical 
strain gauge 




o 


o 


Surface-mount 
photoe la stic gouge 




o 


o 


Surface-mount 
strain gauge 






o 


Surface rosette 
undercoring 






o 


Vibrating wire 
stressmeter 




o 


o 



KEY 
O Data obtainable 

FIGURE 36. - Type of measured data obtained 
from stress measuring instruments. 

point of contact and one direction of 
measurement) or three component (three 
points of contact and three directions of 
measurement) . 

Availability 

Borehole deformation gauges are avail- 
able from the following suppliers: 
Geokon, Inc.; Irad Gage; Rogers Arms and 
Machine Co.; Sinco, Slope Indicator Co.; 
and Soiltest, Inc. The approximate cost 
for the gauge and readout system ranges 
from $1,300 to $3,600. 



30 



Description 

Borehole 
cylinders, 
tilever st 
matched pai 
tached (fi 
gauge has 
cantilevers 
has three 
These gaug 
in-diam hoi 
diam core 
electronic 
for data co 



deformation gauges are round 
1.4 by 12 in, that house can- 
rain transducers to which 
rs of strain gauges are at- 
g. 37). The single-component 
one pair of oppositely placed 
; the three-component gauge 
pairs of such cantilevers, 
es are designed for a 1.5- 
e and overcoring by a 6-in- 
drill (fig. 38). A standard 
strain gauge readout is needed 
llection. 



Installation and Data Collection 

The installation procedure is basi- 
cally the same for the single- and 
three-component gauges, but the single- 
component gauge must be overcored three 
times within the same hole at 120° 
orientations . 



Step 1: Drill a 1.5-in-diam hole to 
the desired depth. 

Step 2: Position the gauge in the 
hole. 

Step 3: Connect the gauge lead wire to 
the strain readout box. 

Step 4: Read the instrument. Be sure 
to read out all three components when a 
three-component gauge is used. 

Step 5: Overcore the gauge with the 
6-in overcore drill while simultaneously 
recording the deformations and depth of 
overcore. 

Step 6: When the gauge is completely 
overcored, remove the core drill, and 
read and remove the gauge. 

(For single-component gauges, repeat 
above installation and data collection 
steps two more times, rotating the gauge 
120° each time.) 

Step 7: Remove the core and store se- 
curely if the core is to be tested in the 
laboratory. 




Scale, in 



KEY 

/ Lug to engage placement tool 

2 Sleeve for placement tool 

3 Cap for cable clamp 

4 Rubber grommet 

5 Body of gauge 

6 0-ring seals 

7 Clamp block 

8 Transducer strip 

9 Tungsten carbide wear button 

10 Piston cap 

// Shim washers 

12 Piston base 

13 Cover of gauge 



LONGITUDINUAL SECTION A- A 

-12 




PISTON ASSEMBLY 
DETAIL 

'/ 2 
I i I 




SECTION 



Scale, in 
FIGURE 37. - Three-component borehole deformation gauge (]_). 



31 




FIGURE 38. - Cross section through a borehole 
showing borehole deformation gauge after over- 
coring (2). 

Data Interpretation 

Stress in the plane normal to the bore- 
hole can be calculated using the follow- 
ing equations (1): 



P + Q = 



3d(1 - y2) (u 1+ u 2 +u 3 ) t 



P - Q = 



E /2 



6d 



5 i L f(u - U ) 2 

(1 - y 2) KU 1 V 



and 
where 



and 



+ (U 2 - U 3 )2 + ( U]L - U 3 )2]l/2, 

Tan 29 ■ - ^ (U 2- U 3^ , 
2Ui - U 2 - U 3 ' 

D lf U 2 , 

and U 3 = the three borehole de- 
formation measure- 
ments, uin/in, 

E = Young's modulus of 
rock, psi, 

y = Poisson ratio of rock, 

d = diameter of the pilot 
borehole, in, 

P = maximum secondary prin- 
cipal stress, psi, 

Q = minimum secondary prin- 
cipal stress, psi, 

= angle from U^ to P. 



If a three-dimensional state of stress 
is desired, deformation measurements must 
be made in three nonparallel boreholes. 
Some of the recommended borehole config- 
urations are shown in figure 39. 




FIGURE 39. - Recommended borehole configurations 
for complete, three-dimensional, state-of-stress deter- 
mination. These various configurations will all reveal 
the same state of stress {]). {D is diameter of mine 
opening.) 

Borehole Inclusion Stressmeter 

Principle and Application 

Borehole inclusion stressmeters are 
rigid devices with an elastic modulus 
greater than that of the material to 
be tested (2^ pp. 363-383). When insert- 
ed into a borehole, a stressmeter mea- 
sures directly the stress changes of 
the surrounding rock. It is secured 
in the borehole by hydraulic pressure 
or with grout and can be left in 
place for long-term stress monitoring. 
Stress is detected by pressure changes 
on a diaphragm, which are transformed 
by a transducer. The transducer can 
be an electrical or foil resistance 
strain gauge or an electrical coil imped- 
ance, photoelastic, or magnetostriction 
device. 

Availability 

The availability of this instrument is 
limited. The cost ranges from $800 to 
$3,000. 

Description 

The borehole inclusion stressmeter 
consists of a rigid outer shell, 



32 



fluid-filled diaphragm or magnetic coil, 
transducer, and readout box. Stress- 
meters have been developed by many in- 
dividuals and organizations (3_) (figs. 
40-42). 

Installation 



Step 1: Drill a 1.5-in-diam hole to 
the desired depth. 

Step 2: Install the stressmeter in the 
hole. If a hydraulic stressmeter is 
used, pump pressure up to that speci- 
fied by the manufacturer. If a grouted 
stressmeter is used, fill the hole with 
grout, then slide the stressmeter to the 
desired depth. 



Step 3: After the stressmeter is set, 
take an initial reading, which will serve 
as reference for all subsequent readings. 




KEY 



/ Blade or core 

2 Slit 

3 Tapered sleeve 

4 Slots 

5 Diaphragm 

6 Electric resistance strain gauge 

7 Insulated sealing disk 

8 Lead wire 



SECTION A- A 



Scole.in 



FIGURE 40. - Cross section of stressmeter (3). 




FIGURE 41. - Stressmeter and tapered sleeve into which it fits (3). 



Cement 



Glass inclusion 



Polarizing material 

I , Reflecting paint 

■^-wave plate 







Lead through hole in 
inclusion to 
light source 



Borehole 
diameter 

'8 



j 



Light source cast 
in plastic 



FIGURE 42. - Cross-sectional view of a borehole showing installed photoelastic stressmeter (3). 



33 



Data Collection 

For nonphotoelastic stressmeters , read- 
ings are taken by connecting the lead 
wires from the stressmeter to a read-out 
box. The photoelastic type of stress- 
meter is read with a polarizer light, 
which illuminates the stress fringe pat- 
terns. Data should be collected on a 
regular basis, preferably once a week. 

Data Interpretation 

Stress can be determined from the re- 
corded strain, using Hooke's law: 



where 



and 



6 = eE, 

6 = stress, psi, 

e = strain, in/in, 

E = Young's modulus of rock being 
tested, psi. 9 



This stress value can be compared 
with the laboratory experimental stress 
determined from a sample of the mate- 
rial to see if the material is near 
failure. 

Borehole-Mount Strain Gauge 

Principle and Application 

This instrument is mounted to the flat 
end of a borehole, then overcored and 
removed to relieve the strain (stress). 
It can be of the electric-resistance or 
photoelastic type. This gauge is used 
primarily to measure strain in pillars 
but can also be used in mine roof and 
floor. 

Availability 

This gauge can be purchased from many 
suppliers, including Micro-Measurements. 
The approximate cost of the gauge and 
readout system ranges from $30 to $1,900. 

^Average values for E are 

Coal: 0.1*10 6 < E < 1.5x10 6 psi. 



Description 

This instrument is simply a single 
strain gauge or rosette that is glued to 
the flattened end of a borehole. In 
addition to the gauge, the user needs 
setting glue, setting tools, overcore 
drill, flat-grinding drill, and readout 
equipment. 

Installation 



2.0*10 6 < E < 15*10 6 psi. 



? o ~> : 



Steel: E * 30x10° psi, 



Step 1: Drill a 1.75-in-diam hole to 
the desired measuring depth (usually in a 
pillar) . 

Step 2: Grind the bottom end of the 
hole flat. 

Step 3: Read the gauge before 
installation. 

Step 4: Glue gauge to the flat surface 
of the hole. Orient gauge so that one 
component is axially lined up with the 
suspected direction of principal stress 
(strain) . 

Step 5: Overcore gauge 6 to 12 in, 
then remove core drill and core. 

Step 6: Read the gauge. 

Data Collection 

Data are collected twice during the in- 
stallation process: The gauge is read 
just prior to being overcored and after 
it has been overcored and the core and 
gauge have been removed from the hole. 
These readings, as well as the date, 
time, installation location, depth of 
gauge in hole, and person(s) taking the 
readings and involved in the installation 
process should be recorded in a field 
book. 

Data Interpretation 

The strain in the pillar is found by 
subtracting the original reading from the 
overcored reading. Stress is calculated 
using Hooke's law: 

6 = eE. 

Since stress is measured inside the 
pillar, it can be compared with the 
yield strength of the pillar to deter- 
mine if the pillar is near failure. 



34 



The pillar 
follows: 



strength is determined as 



where S = estimated strength of pillar, 
psi, 

T = seam thickness, in, 

L = least lateral pillar dimen- 
sion, in, 

K = S L /W[, psi, 

W L = cubic size of lab specimen, 
in 3 , 

and S L = strength of lab specimen, 
psi. 

CSIR Strain Gauge Strain Cell 
(Doorstopper) 

Principle and Application 

The Doorstopper Cell, developed by the 
Council for Scientific and Industrial Re- 
search (CSIR), is designed to determine 
the absolute stress in rock using an 
overcoring stress-relieving technique. 
It may, however, be used to measure 
changes in stress, provided the glue used 
to bond the cell possesses sufficiently 
stable characteristics. The cell mea- 
sures the major principal stress where 
its direction and those of the two other 
principal stresses are known or can be 
assumed. It is also possible to deter- 
mine the complete state of stress using 
Doorstoppers by taking measurements in 
three boreholes drilled in any three dif- 
ferent and known directions (4). 



angles, molded into a rubber casting that 
fills a plastic shell. The gauges are 
connected by lead wires to a strain- 
indicating instrument. The cell can be 
used in either a BW (2.4-in nominal diam) 
or NW (3.0-in nominal diam) diamond- 
drilled borehole. The rubber and plastic 
shell serve to protect the strain gauges 
from damage and from water during the 
overcoring operations (4). 

Installation and Data Collection 

Step 1: A temperature compensation 
dummy cell with a 1/2-in length of BX 
(1.5-in nominal diam) core attached must 
be installed in the installation tool 
prior to use in the mine. 

Step 2: Drill a borehole to the de- 
sired measurement zone. Grind flat and 
polish the end of the borehole. 

Step 3: Plug a Doorstopper Cell into 
the installation tool, cover cell with 
glue, then push cell to back of the bore- 
hole and orient cell as indicated by ori- 
entation device. 

Step 4: Push cell against rock until 
correct pressure is obtained. Wedge the 
rods in place to maintain this pressure 
until glue hardens. 

Step 5: After the glue has hardened, 
read out the strain in the cell, using a 
standard strain gauge indicator unit, and 
then remove the installation tool from 
the borehole. 

Step 6: Overcore cell with the same 
size core barrel to a minimum depth of 6 
in. Break off core and remove from the 
borehole. 

Step 7: Plug 
stallation tool 
relieved readings, 
laboratory testing. 



cell back 


into 


the in- 


and take 


the 


stress- 


s . Save 


the 


core for 



Availability 

The Doorstopper Cell is available from 
Roctest, Inc. , at a cost of approximately 
$56 per cell. A complete system will 
cost approximately $7,500. 

Description 

The Doorstopper consists of four strain 
gauges at -45°, 0°, +45°, and +90° 



Data Interpretation 

The magnitude and direction of stress 
can be calculated from the data using the 
following equations (40: 

Magnitude: 



' a l = 



- y 2 



(e l + ye 2 ) 



35 



and 
where 



a„ = 



2 1 - u 2 



(e 2 + pej), 



o± = magnitude of maximum horizontal 
stress at end of borehole, psi, 

0"2 = magnitude of minimum horizontal 
stress at end of borehole, psi, 

E = Young's modulus of rock, psi, 

\i = Poisson ratio of rock, 
e^ = maximum principal strain, pin/in, 
e 2 = minimum principal strain, yin/in, 
1,2 " I/? |(e H + e v ) 



±/[2e 45 -(e H + e v )] 2 + (e H -e v ) 2 ], 



and 

e H> e 4 5- 



and 



difference in strain readings 
in the horizontal, 45°, and 
vertical directions before 
and after overcoring, re- 
spectively, yin/in. 

r, = 1/1.53 a[ 
2 ~ 1/1.53 a 2 » 



where a^ = magnitude of maximum horizon- 
tal stress, psi, 

and o"2 = magnitude of minimum horizon- 
tal stress, psi. 

Direction: 

Tan e, - , 2 (£l "V V 
2e 45 " ( e H + e v ) 



6? = 



2 (e 2 - e H ) 



2 '" 2e 45 - (e H + e v )' 

where Q± is the angle measured anticlock- 
wise from the horizontal (e H ) direction. 
The determination of the stress is im- 
portant in planning of mine design, 



layout of panels, and analysis of proba- 
ble areas of ground control problems. 

CSIR Triaxial Strain Cell 

Principle and Application 

The CSIR triaxial strain cell is de- 
signed to obtain the complete state of 
stress in rock in a single borehole (5). 
The change in strain associated with the 
overcoring stress-relief method is de- 
tected by the strain gauges mounted in 
the instrument body. 

Availability 

This instrument is available from Roc- 
test, Inc. Cost of the cell, installa- 
tion equipment, and readout boxes is ap- 
proximately $6,000. 

Description 

The cell consists of a plastic housing 
containing three strain gauge rosettes 
mounted on pistons, which are subsequent- 
ly glued to the borehole walls. The 
pistons are actuated by air pressure. 
Strain changes are read using a standard 
strain gauge indicator unit. 

Installation and Data Collection 

Step 1: Drill a 3.5-in-diam hole to 
the depth at which the stress is to be 
determined. 

Step 2: Drill a 1.5-in-diam hole for 
18 in into the end of the borehole. 

Step 3: Cover the three strain gauge 
rosettes with glue, insert the cell into 
the hole, then turn on the air pressure 
to actuate the pistons. 

Step 4: After the glue has set, turn 
off the air pressure and take the inital 
strain readings using a standard strain 
gauge indictor. 

Step 5: Overcore the cell with a 3.5- 
in-diam core barrel. 

Step 6: Remove the core, plug back in- 
to the installation tool, then take a 
second set of strain readings. 



36 



Data Interpretation 

The complete state of stress is deter- 
mined from the strain readings using the 
following equations: 



Magnitude: 



E 



e A + e 



C B + e A e B 



T+IT 



and 



°B 



AB 



e A + e B _ e A ~ e B 



2 L l - y 



1 + y J 



2e c - (e A + e B ) 



where a A = normal stress in A direction, 
psi, 

a B = normal stress in B direction, 
psi, 

T AB = tangential stress, psi, 

E = Young's modulus of rock, psi, 

y = Poisson ratio of rock, 



e A = measured strain in A direc- 



tion (X-axis), yin/in, 

leasured strain in B di 
tion (Y-axis), yin/in, 



e B = measured strain in B direc- 



and 



e c = measured strain in C direc- 
tion (45° to A and B), 
yin/in. 



Direction: 

Tan A = 



and 



Tan 9 B = 



2(e, - e A ) 
2e c - (e A + e B ) 

2(e 2 - e A ) 
2e c - (e A + e B )' 



where 9 A is the angle measured anticlock- 
wise from the horizontal direction (X- 
axis) , 

and e, >2 = 111 J (e A + e B ) 

± /(e A - e B ) 2 + [2e c -(e A + e B )] 2 |. 



CSIRO Hollow Inclusion Stress Cell 

Principle and Application 

The CSIRO cell, developed by the 
Commonwealth Scientific and Industrial 
Research Organization (CSIRO), provides 
a method of determining the three- 
dimensional stress state in rock or coal 
(60. Strain gauges mounted in the in- 
strument measure the change in strain as 
the rock "relaxes" after overcoring. The 
cell can also be left in place for long- 
term monitoring of stress changes. 

Availability 

This instrument is available from 

Geokon, Inc. , at a cost of approximately 

$500 per cell, $6,000 for a complete 
system. 

Description 

The CSIRO cell consists of a fully 
encapsulated array of nine strain gauges 
mounted in the instrument body. The 
cell is 0.4 in long and is grouted into 
a 1.5-in-diam hole. It is constructed 
from epoxy pipe with the gauges precise- 
ly oriented at 120° angles along the 
circumference. 

Installation and Data Collection 

Step 1: Drill a 1.5-in-diam hole to 
the desired measurement zone. 

Step 2: Fill the hole with grout, then 
insert cell and push to back of hole, 
extruding the grout. 

Step 3: Allow the grout sufficient 
time to harden. 

Step 4: Readout the strain on all the 
gauges. 

Step 5: Overcore the cell with a 6- 
in-diam core barrel. Monitor the strain 
response during overcoring. 

Step 6: Read out the new strain on all 
the gauges. 

Data on the strain in the rock as indi- 
cated by cell readings are obtained using 
a nine-channel switch box and quarter 
bridge strain indicator. 



37 



Data Interpretation 

A data reduction program supplied by 
the manufacturer analyzes the stress 
tensor using the overcore strain data 
and biaxial pressurization results, giv- 
ing the stress state in both principal 
form and oriented along coordinate axes 
(6). The stress information can be used 
in mine design studies and geotectonic 
studies. 

Cylindrical Borehole Pressure Cell 

Principle and Application 

This device determines the modulus of 
rigidity of rock or coal by direct 
measurement inside a small-diameter bore- 
hole. It measures the change in volume 
of the borehole with respect to the ap- 
plied pressure, which is interpreted by 
thick-wall cylinder equations for an 
elastic body (_7_) . 

Availability 

Cylindrical borehole pressure cells 
can be made in-house, or purchased from 



Sinco, Slope Indicator Co. The approxi- 
mate cost is $350. 



Description 

The cylindrical borehole pressure cell 
consists of a steel core and copper jack- 
et. Overall length is 8 in. The cell is 
installed in a 1.5-in-diam hole without 
grout. In-mine tests require a hydraulic 
pump, fluid reservoir, and gauge. A cal- 
ibration cylinder is needed to calibrate 
the cell before installation. 

Installation and Data Collection 

Step 1: Drill a hole to the desired 
depth. The hole must be 1.5 in, at the 
measuring zone, and must not have open 
joints or cracks wide enough for the cop- 
per shell to extrude into them. 

Step 2: Slide cell into hole. No spe- 
cific orientation is required because 
cell is equally sensitive to changes in 
all directions. 

Step 3: Connect pressure gauge and 
fluid pump to cell and fluid reservoir 
(fig. 43). 




/ Cylindrical borehole pressure cell 

2 Hydraulic tubing 

3 Wedges 

4 Fluid pump 

5 Pressure gauge 



Scale, ft 

FIGURE 43. - Typical installation of a cylindrical borehole pressure cell (]_). 



38 



Step 4: Completely fill cell with flu- 
id, then bleed off any air in the system. 
Only slight pressure is needed. 

Step 5: Cell is ready for pressure cy- 
cling, consisting of loading, unloading, 
and reloading at a rate of 200 psi/min, 
with readings taken at 1-min intervals. 
The measurements taken are the volume of 
fluid injected into the cell versus the 
cell pressure reading. The test for the 
modulus of rigidity is complete after the 
second loading. Maximum pressure per 
loading is 3,500 psi. 

Step 6: Cell can now be removed by re- 
leasing pressure. Measurements can be 
repeated at another horizon within the 
hole. Cell can also be left in place to 
continue monitoring. In this case, pres- 
sure in cell must be continued by closing 
valve located between cell and hydraulic 
pump. Pump and fluid reservoir can now 
be removed ( 1 ) . 

Readings need to be taken only during 
the test procedure described in step 5 
above. If the cell is left in place, it 
should be checked once a week. 



2. Insert the cell into the calibra- 
tion cylinder and determine the slope of 
the experimental pressure-volume curve 
(M m ). This is done by pressure cycling 
as described in the installation proce- 
dures, step 5. The system stiffness (M s ) 
is calculated by 



M = 
s M„ - M„ 



3. 
cell 
the 
late 
(M r ) from the test data: 



Perform pressure cycling with the 

in the borehole and determine 

pressure-volume curve (M t ). Calcu- 

the pressure-volume relationship 



M t M s 



M = 

r M + - M c 



4. The modulus of rigidity for the 
rock is calculated from 



G = M r tt &r; 2 



Flat Borehole Pressure Cell 



Data Interpretation 



Principle and Application 



The modulus of rigidity (G r ) can be de- 
termined as follows (1_) : 

1. Calculate M c for a calibration cyl- 
inder of known properties using 



M c = 



vG 



TT IVy 



1 + B - 2 vB 
1 - B 



A flat borehole pressure cell is a 
flatjack, 2 in wide, 8 to 10 in long, and 
0.125 to 0.25 in thick ( _7) , designed for 
permanent (long-term) installation. It 
measures the changes in pressure from 
continued mining. It can be installed in 
the roof, floor, and ribs and is either 
grouted in place or preencapsulated. 



where v = volume per turn of pressure 
generator, in 3 , 

I = effective length of pressure 
cell, in, 



and 



B = 



= — L , where r } and r Q are 

the inner and outer radii of 
the cylinder, 



G = modulus of rigidity of cali- 
bration cylinder, psi. 



Availability 

This instrument can be made in-house 
or purchased from Geokon, Inc. , or Sinco, 
Slope Indicator Co. The approximate cost 
ranges from $195 to $230. 

Description 

The flat borehole pressure cell con- 
sists of a thin-walled, fluid-filled met- 
al bladder, hydraulic connection lines, 
and a pressure gauge (fig. 44). It is 



39 




FIGURE 44. - Steps in fabrication of an encapsulated flat borehole pressure cell. A, Copper tube 
cut to length; B, tube flattened to 1 4-in opening; C, top half of ends cut; J), top half of ends re- 
moved; E, bottom of ends folded up and brazed to top; F, fluid-filling and pressuring and gauge tubes 
attached; 0, cell encapsulated in plaster. 



either preencapsulated with a cement mix- 
ture or grouted in place. A hydraulic 
hand-operated pump is required to set 
the instrument at the correct insertion 
pressure. 

Installation 

The procedure depends on the type of 
cell used. Preencapsulated cells require 
a precise hole but can be installed, 
pressurized, and read within 30 to 60 
min. Grouted cells do not require a pre- 
cise hole, but pressurization and moni- 
toring cannot begin until the grout has 
set. 

Step 1: Drill a hole to desired depth. 
Hole diameter is about 2.25 in. 



Step 2: Insert cell and orient it 
within the hole. Grout in place if nec- 
essary. Cell measures pressure perpen- 
dicular to its flat side. 

Step 3: Attach pressure gauge and pump 
to cell. A shutoff valve must be located 
between the pressure gauge and pump. 
Create a pressure in the cell equal 
to the known or estimated pressure of 
the surroundings. Hold this pressure by 
closing the shutoff valve. 

Step 4: Record this pressure as read 
from the gauge. 

Step 5: Take readings every few min- 
utes for the first hour to check for 
pressure leaks or other problems. 



40 



Data Collection 

The amount of pressure, as indicated on 
the gauge, should be recorded once a 
week. Any drastic changes should be in- 
vestigated and reported to the appropri- 
ate authorities. 

Data Interpretation 

The information obtained from flat 
borehole pressure cells is the pressure 
induced by mining. To obtain the change 
in pressure, the original (installation) 
pressure must be subtracted from the sub- 
sequent readings. Various ground control 
problems are associated with increased 
pressure. Knowing the allowable pressure 
limits could help eliminate hazards as- 
sociated with squeezing, floor heave, 
bursts, etc. 

Flat jack 

Principle and Application 

A flat jack is a thin-walled fluid- 
filled metal bladder designed to with- 
stand several thousand pounds per square 
inch when confined in a slot in rock 
strata (]_) . It records the pressure 
within the slot. The pressure needed to 
bring the rock back to equilibrium is the 
stress in the rock before the stress is 
relieved. The flat jack method can mea- 
sure rock stress directly. 

Availability 

Flat jacks are usually manufactured in- 
house, but can be purchased from Geokon, 
Inc., or Sinco, Slope Indicator Co. 
The approximate cost of this instrument 
ranges from $100 to $250. 

Description 

A flat jack consists of a metal blad- 
der, hydraulic connection line, and pres- 
sure gauge (fig. 45). For measurement 
purposes , a hydraulic pump and two ref- 
erence pins are needed. The flatjack 
should be sandwiched between flat steel 
plates or encapsulated to ensure uniform 
pressure distribution. 




FIGURE 45. - Flatjack pressure cell. 
Installation and Data Collection 

Step 1: Cement two measurement pins to 
rock surface or grout them in holes 
drilled into the rock. Spacing should be 
1 to 12 in from the slot (2 to 24 in 
apart) . Pins should be perpendicular to 
the flatjack when installed. 

Step 2: Measure the distance between 
pins with as precise a measuring in- 
strument as is available. Record this 
measurement. 

Step 3: Cut a slot into the rock sur- 
face perpendicular to pins by drilling a 
series of overlapping holes. This re- 
lieves stress perpendicular to the slot. 

Step 4: Take and record another read- 
ing across the pins. 

Step 5: Embed flatjack in slot. The 
flatjack can be grouted in the slot 
if desired. Use hand pump to steadily 
increase pressure within flatjack until 
displacement (stress relieved) by the 
cut slot is cancelled. This requires 
frequent measurements across the pins to 
determine when they are the original dis- 
tance apart. This cancellation pressure 
is equal to the original stress in the 
area monitored. 

The information obtained is the can- 
cellation pressure needed to restore 
the area to its original position, which 
represents the original stress in the 
area. Usually, no additional readings 
are taken since the flatjack is removed 
when the process is completed. However, 
if desired, the flatjack can be left 
in place to continue monitoring rock 
stresses. 



41 



Data Interpretation 

Cancellation pressures (original 
stress) can be analyzed to determine 
several possible situations, all deal- 
ing with excessive pillar pressures. 
Excessive pressures (stress) can be the 
result of inadequate pillar size, pillar 
orientation, insufficient barrier pil- 
lars, etc. If it is known what pressures 
are acceptable, mine design can be mod- 
ified, resulting in safer and more stable 
roof and pillars. 

Mechanical Strain Gauge 

Principle and Application 

A mechanical strain gauge determines 
rock stress by the stress relief method. 
It measures the strain between three ref- 
erence points, set in the surface of the 
rock, after a series of relief holes are 
drilled. This gives surface strain only. 
Hooke's law is used to convert the strain 
to the maximum and minimum principal 
stresses. 

Availability 

Mechanical strain gauges are available 
from Soiltest, Inc. The approximate cost 
is $425. 

Description 

This system consists of a mechanical 
strain gauge (fig. 46) with an attached 
dial gauge and three reference (measure- 
ment) pins. A suitable drilling appara- 
tus is also required. 

Installation 

Step 1: Grout three pins into or on 
the rock surface, in a triangular layout. 

Step 2: Measure the strain between the 
pins with the mechanical strain gauge. 
Record the readings. 

Step 3: Drill a series of adjacent 
holes, 12 to 30 in deep and 1.5 to 3 in. 
in diam, completely around the pins. 
This relieves stress and allows the pins 
to move in any direction (fig. 47). 

Step 4: Remeasure and record strain 
between pins using mechanical strain 
gauge. 



Data Collection 

Strain is measured and recorded twice 
during the instrumentation process: (1) 
just after pins are installed and (2) af- 
ter the stress relief holes have been 
drilled. 

Data Interpretation 

Hooke's law is used to derive the 
principal stress. The strain used in 
the equations is the change in strain 
(the second measurement minus the first 
measurement). The allowable stresses be- 
fore failure occurs must be determined 
through observations such as this. These 
stresses will help determine the proper 
entry orientation, pillar size, etc. 



Dial gouge, j^^ in 



Moving f\ 




/Rigid cast 
/ frame 



10 



point - 



— H Fixed point 



FIGURE 46. - Mechanical strain gauge. (8). 




FIGURE 47. - Typical measurement setup forme- 
chanical st rain gauge showing me asuringpoints(l A, 
2B, and 3C) and stress relief holes (8). 



42 



Hooke's law (9) 

S = E 



T = E 



e A + e B + e c + 1 lr _ e A + e B + e c \ 2 + S e c - e B y 

3 (1 ■ y) 1 + y VV A 3 / V /3 / 



e A + e R + e f 



3 (1 - y) 

<J) = 1/2 tan" 1 [ J. 
/3 



(e c " e B ) 



e A + e R + e r 



where e A , e B , e c = 



S = 



T = 



E = 



strain at 60° orienta- 
tions, yin/in, 

maximum principal 
stress, psi, 

minimum principal 
stress, psi, 

Young's modulus of 
rock, psi, 



and 



y = Poisson ratio of rock, 

<() = angle from maximum 
principal stress to 
A- axis. 



Surface-Mount Photoelastic Gauge 

Principle and Application 

Surface-mount photoelastic gauge mea- 
surements are taken by mounting thin 
sheets of photoelastic material to the 
rock, coal, or prop surface. This photo- 
elastic coating reacts exactly as the 
surface does, and by its photosensitive 
nature reveals stress changes on the ma- 
terial surface (10) . The stress patterns 
that develop can be interpreted using a 
polarizer. 

Availability 

Surface-mount photoelastic gauges are 
available from Micro-Measurements. The 



approximate cost of the gauge and readout 
system ranges from $100 to $500. 

Description 

Photoelastic gauges are of two types: 
(1) sheets of materials that are glued to 
the surface and (2) sprayon or brushon 
paint (these may be extremely difficult 
to use in the underground environment). 
Fringe patterns develop that reveal the 
stress changes on the surface of the ma- 
terial. A polarizer and viewer are used 
to detect the fringe patterns. 

Installation 



Step 1 : Prepare the surface by chip- 
ping or grinding it as smooth and flat as 
possible. Surface preparation is criti- 
cal for a good installation. 

Step 2: Glue the sheet-type gauge or 
paint the photoelastic coating on the 
surface. The glue or coating dries in a 
few minutes. Stress changes are now be- 
ing detected and displayed as fringe pat- 
terns in the photoelastic material. 

Data Collection 

Data can be collected as often as de- 
sired and for as long as desired. The 
polarizer and viewer are used to take 
readings from the photoelastic gauge in 
the following manner. First, the polar- 
izer is used to illuminate and define the 
fringes. The handle of the handheld 



43 



viewer is held in line with the loading 
axis of the gauge. The aperture on the 
viewer has a rotating compensation scale. 
With the scale initially set at zero, the 
number of visible photoelastic fringes 
(fig. 48) in one-half of the glass cylin- 
der is counted. An exact count is made 
when the fringe at the center forms a 
cross (X). Where this is not the case, 
the full fringes are counted, and then 
the compensation scale is rotated clock- 
wise until the last fringe counted 
(nearest the center) has moved back to 
form a cross. In this case, the scale 
reading is added to the initial full 
fringe count. The fringe count is multi- 
plied by the appropriate cell sensitivity 
factor to obtain the change of stress 

on. 

Data Interpretation 

The stress changes shown depend on the 
type of photoelastic material used, as 
well as on the object monitored. Each 
material has its own analysis graph, 
which will convert fringe patterns to 
stress values. Each material monitored 
has specific stress limits that must not 
be reached, or failure will occur. 

Surface-Mount Strain Gauge 

Principle and Application 

Surface-mount strain gauges are mount- 
ed on the flat surface of rock, coal, 
or props and detect the strain or stress 
changes on the surface of the materi- 
al (12) . The overcoring stress relief 
method can be used, or the gauges can 
be left in place for long-term monitor- 
ing. There are three types of gauges: 
electric resistance, vibrating wire, and 
photoelastic. 

Availability 

Strain gauges can be purchased from the 
following suppliers: BLH Electronics; 





1/2 fringe 



I fringe 





2 fringes 



2-1/2 fringes 





4 fringes 



4-1/2 fringes 






m 





5-1/2 fringes 6 fringes 

FIGURE 48. - Example of photoelastic fringe 
patternsdisplayed by a surface-mount photoelas- 
tic gauge (TJ). 



44 



Budd Co.; Geokon, Inc.; Hitec Corp.; 
Irad Gage; Kulite Semiconductor Products, 
Inc.; Microdot, Inc.; Micro-Measurements; 
Sinco, Slope Indicator Co.; and Soiltest, 
Inc. The approximate cost of the gauge 
and readout system ranges from $25 to 
$1,850. 

Description 

The electric-resistance gauge measures 
resistance changes in wires of the gauge 
as wire length changes. The vibrating- 
wire gauge measures vibration frequency 
as wire length changes (fig. 49). The 
photoelastic gauge measures stress by 
showing fringe patterns. All three types 
are attached to the flat surface of 
rock, coal, or props. They can be glued, 
grouted, or welded on (steel props only). 
If the stress relief method is used, an 
overcoring drill is needed. Each system 
has a matching readout system. 

Installation 



Installation is the same for the 
three types of gauges. The following 
procedure is for the overcoring method 
(fig. 50): 

Step 1: Pick desired location, then 
make the surface as smooth and flat as 
possible. 

Step 2: Glue, grout, or weld the gauge 
to the flat surface. 

Step 3: After the gauge is set, take a 
reading. 

Step 4: Overcore the gauge using a 
core drill that is larger than the gauge. 
Drill 6 to 12 in deep into material. 

Step 5: Remove core drill. Break off 
core at back of hole and remove. 

Step 6: Read the gauge. Subtract this 
reading from the original reading to de- 
termine strain in the material before it 
was relieved by overcoring. 




FIGURE 49. - Vibrating wire surface-mount 
strain gauge (13). 




Stress relief 
diamond bit kerf 



FIGURE 50. - Surface-mount strain gauges show- 
ing stress relief using large overcoring bit (9). 



45 



For a permanent gauge installation (no 
overcoring) , the procedure is as follows: 

Steps 1 through 3: Same as above. 

Step 4: Coat gauges with sealer. 

Step 5: Take readings at predetermined 
intervals to determine change in strain 
as mining continues. 

Data Collection 



Availability 

A mechanical strain gauge or Whit- 
more gauge for making the measurements 
is available from Soiltest, Inc., and 
costs approximately $1,000. The ref- 
erence pins, template, and masonry bits 
can be purchased for under $100 per 
setup. 



Electrical-resistance gauges and vi- 
brating-wire gauges are read by connect- 
ing the readout box to the gauge with a 
cable, then recording the reading that 
is displayed. Reading the photoelastic 
gauge requires a polarizer light and 
viewer to detect the stress fringe pat- 
tern (see the section "Data Collec- 
tion" for the surface-mount photoelastic 
gauge). Readings are taken twice during 
the overcoring process, and at desired 
intervals (usually once a week) during 
long-term monitoring. 

Data Interpretation 

The overcoring process reveals the to- 
tal strain in the medium (pillars, roof, 
etc. ) , whereas leaving the gauges in 
place reveals the change in strain in- 
duced by mining activity. The total 
strain indicates if the medium is near 
failure. The change in strain indicates 
the rate of loading leading to failure. 
This information is valuable in achieving 
optimum safety through efficient mine 
design. 

Surface Rosette Undercoring 

Principle and Application 



Surface rosette 
the two-dimensional 
the collar of a bor 
tween three sets of 
face of the rock i 
after a large reli 
Hooke's law is used 
measurements to the 
cipal stresses. 



undercoring determines 
state of stress about 

ehole. The strain be- 
pins set in the sur- 

s measured before and 

ef hole is drilled, 
to convert the strain 
two-dimensional prin- 



Description 

The system consists of a mechanical 
strain gauge, reference pins, and tem- 
plate. A suitable drilling apparatus is 
also required. 

Installation 



Step 1: Use the template to mark po- 
sitions of reference pins. Drill holes, 
and grout pins in place. 

Step 2: Take a strain reading across 
the pins. 

Step 3: Drill a 24-in-deep hole with 
a 6-in-diam core barrel. Remove and save 
core for laboratory testing of rock 
properties. 

Step 4: Take another strain reading 
across the pins (14). 

Data Collection 

Readings are taken just prior to under- 
coring and just after stress relieving by 
undercoring. 

Data Interpretation 

Hooke's law is used to derive the 
principal stress. The strain used in 
the equations is the change in strain 
(the second measurement minus the first 
measurement). The allowable stresses be- 
fore failure occurs must be deter- 
mined through observations such as this. 
These stresses will help determine the 
proper entry orientation, pillar size, 
etc. 



46 



Hooke's law (9): 
S = E 



L 3 ( 



e B + e c _J_ // . e A + e B + e c \ 2 fjcZJjjS 
1-y) 1 + y H 3 / V /J / 



T = E 



e A + e R + e 



B Z e C _ 



3 (1 - y) 1 + y 



<|> = 1/2 tan 



-1 



/5 (ec " ee) 



e A - 



e A + e R + e, 



/3 



e A _ e A + e B + e c y + 



where e A , e B , e c = 



S = 



T = 



E = 



strain at 60° orienta- 
tions, pin/in, 

maximum principal 
stress, psi, 

minimum principal 
stress, psi, 

Young's modulus of 
rock, psi, 



and 



y = Poisson ratio of rock, 

<j> = angle from maximum 

principal stress to 
A-axis. 



Vibrating Wire Stressmeter 

Principle and Application 

A vibrating wire stressmeter is an 
instrument designed to monitor stress 
changes within mine rocks. It measures 
the altering period of the resonant fre- 
quency of a highly tensioned steel wire 
clamped diametrically across the gauge 
( 13 , 15) . This reading is converted to 
pressure (pounds per square inch) using 
the appropriate calibration graph. 

Availability 

This instrument is available from 
Geokon, Inc. , or Irad Gage. The approxi- 
mate cost of the stressmeter and readout 
system ranges from $150 to $6,000. 

Description 

The vibrating wire stressmeter con- 
sists of a highly tensioned steel wire 



surrounded by a metal housing, anchorage 
platen, anchorage wedge, and electrical 
wire (fig. 51). A readout box is used to 
collect data. 

Special setting tools and rods 
(manual or hydraulic) are needed for 
installation. 

Installation 



Step 1: Drill a 1.5-in-diam hole to 
desired depth. Diameter range is 1.475 
to 1.525 in for hard rock and 1.450 to 
1.545 in for soft rock and coal. 

Step 2: Thread the loose end of wire 
from the stressmeter through the hole 
in setting tool. Place the stressmeter 
and platen in place on the end of the 
setting tool. Using a hydraulic pump, 
slowly apply pressure until the stress- 
meter is held in place. Take initial 
reading. 

Step 3: Slide the stressmeter and set- 
ting tool into hole. Align the stress- 
meter to desired measuring plane in hole. 
Connect needed setting rods onto assembly 
as it is slid into hole. 



*-»<^^» <f «-* ; **"' I 




Scale, in 



FIGURE 51. - Vibrating wire stressmeter. 



47 



Step 4: When the stressmeter is at the 
desired depth, pump pressure into the 
system until the stressmeter readout is 
300 units above the original reading. 
Once this reading is reached, the stress- 
meter is securely anchored. 

Step 5: Slowly let the pump pressure 
off. The rods should be left free so 
that they can move slightly back out of 
the hole. When the pressure has reached 
zero and the rods have stopped mov- 
ing (approximately 1 min) , the assembly 
can be slowly pulled out of the hole and 
disassembled. 

An additional two stressmeters can be 
installed in this hole. To avoid dam- 
age, care must be taken to properly 
route the previously installed stress- 
meter wires, as well as the wire of the 
stressmeter being installed, through the 
setting tool guide slots. More specific 
instructions can be obtained from the in- 
strument supplier. 

Data Collection 

Vibrating wire stressmeters are read 
twice during installation, then at de- 
sired intervals thereafter. For a de- 
tailed account of pressure changes, a 
data logger can be used. It will provide 
a continuous tape readout and can be set 
to read as often as desired. 

Data Interpretation 

Readout data are numbers that indicate 
the frequency of vibration of the wire. 
A calibration chart is needed to change 
these data into pounds per square inch. 
When the pressure is known, such param- 
eters as roof or rib stability can be 
analyzed and used for design modifica- 
tions, such as orientation and pillar 
size. 

REFERENCES 

1. U.S. Bureau of Mines. Rock Mechan- 
ics Instrumentation for Mine Design. 
Proceedings: Bureau of Mines Technology 
Transfer Seminar; Denver, Colo.; July 25, 
1972, comp. by Technology Transfer Group, 
Office of the Assistant Director — Mining. 
IC 8585, 1973, 76 pp. 



2. Jaeger, J. C. , and N. G. W. Cook. 
Fundamentals of Rock Mechanics. Methuen 
& Co. Ltd. , 1969, 513 pp. 

3. Leeman, E. R. The Measurement of 
Stress in Rock, Part II: Borehole Rock 
Stress Measuring Instruments. J. S. Afr. 
Inst. Min. & Metall. , v. 65, No. 2, 1964, 
pp. 82-114. 

4. Roctest, Inc. (Plattsburgh, NY). 
Instruction Manual, CSIR Strain Gage 
Strain Cell ("Doorstopper"). 1984, 17 pp. 

5. Leeman, E. R. The "Doorstopper" 
and Triaxial Rock Stress Measuring In- 
struments Developed by the C.S.I.R. J. 
S. Afr. Inst Min. & Metall., v. 69, No. 
7, 1969, pp. 305-339. 

6. Geokon, Inc. (West Lebanon, NH) . 
Geotechnical Instrumentation. Tech. bro- 
chure, 1983, 2 pp. 

7. Wang, C. Survey of Tools and 
Techniques for Roof Control Studies in 
Underground Coal Mines. BuMines , PMSRC 
Interim Report, Mar. 1972, 43 pp.; avail- 
able upon request from E. R. Bauer, Bu- 
reau of Mines, Pittsburgh, PA. 

8. Parker, J. The Relationship Be- 
tween Structure, Stress and Moisture. 
Eng. and Min. J., v. 174, No. 10, 1973, 
pp. 91-95. 

9. Griswald, G. B. How To Measure 
Rock Pressures: New Tools and Proved 
Techniques Aid Mine Design. Eng. and 
Min. J., v. 164, No. 10, 1963, pp. 90-95. 

10. Reed, J. J. Survey of Develop- 
ments in the Field of Rock Mechanics. 
Min. Eng. (N.Y.), v. 14, No. 4, 1962, 
pp. 60-62. 

11. Terrametrics (Golden, CO). Photo- 
elastic Prop Load Cell. Bull. 21.21, 
1968, 2 p. 

12. Morgan, T. A., W. G. Fischer, and 
W. J. Sturgis. Distribution of Stress in 
the Westvaco Trona Mine, Westvaco, Wyo. 
BuMines RI 6675, 1965, 58 pp. 

13. Irad Gage (Lebanon, NH) . Geotech- 
nical Instrumentation. Catalogue, 1980, 
40 pp. 

14. U.S. Bureau of Mines. Economical 
Method for Determining Stress in Mines. 
Technology News, No. 96, 1981, 2 pp. 

15. Irad Gage (Lebanon, NH) . Vibrat- 
ing Wire Stressmeter Instruction Manual. 
1980, 33 pp. 



48 



CHAPTER 5. — SUPPORT LOAD MEASUREMENTS 



INTRODUCTION 

Support load is the weight exerted on 
a roof support (post, crib, roof bolt, 
etc.) by the roof and overburden. Mea- 
surement of support loads can reveal pos- 
sible support and/or roof failures. 

Support load measurements provide mine 
operators with the means of detecting 
ground (roof, rib, and floor) control 
problems before they occur, analyzing 
their cause and preventing further occur- 
rences , and making necessary changes in 
mine design. 

GROUND MOVEMENT INDICATORS 

Like other ground movements , support 
load has specific indicators that char- 
acterize it. Unfortunately, some indi- 
cators are common to several movements. 
Therefore, differentiating among sev- 
eral movements can be difficult and at 
times may be just an educated estimate. 
The following is a list of support load 
indicators: 

• Excessive number of roof falls. 

• Extreme or continual torque loss on 

roof bolts. 

• Broken posts. 

• Roof bolts shearing and then falling 

out of hole. 

• Loss of bolt anchorage. 

• No bolt anchorage when installing. 

• Inability to torque bolts at 

installation. 

• Squeezing cribs. 

• Downward-bending roof support beams . 

INSTRUMENT SELECTION 

As requirements and parameters are de- 
fined, they should be listed on an in- 
strument selection worksheet (fig. 1). 
Since this chapter deals with support 
load measurements only, support load 
should be listed in the "measurement de- 
sired" column. The controlling parame- 
ters, as defined in the section "Instru- 
ment Selection Guidelines" should be 
listed in the "desired elements" column. 

Next, the user should refer to fig- 
ures 52 through 54 and follow steps 1 



through 6 to make a preliminary choice 
of the most. suitable instrument(s) 
for the ground control problem being 
investigated. 

Step 1: All of the instruments in 
this chapter satisfy the "measurement de- 
sired" requirement (support load) previ- 
ously listed on the worksheet. List 
these instruments under "instruments 
available," across from "measurement 
desired." 

Step 2: From figure 52, choose the in- 
struments that satisfy the cost parameter 
previously selected and list them under 
"instruments available," across from the 
cost parameter. 

Step 3: From figure 53, choose the in- 
struments that satisfy the technical as- 
pects parameter selected and list them 
under "instruments available," across 
from the technical aspects parameter. 

Step 4: From figure 54, choose the in- 
struments that satisfy the data require- 
ment parameter and list them under "in- 
struments available," across from the 
data requirement parameter. 

Step 5: From the "instruments availa- 
ble" column, determine the instruments 
that satisfy all requirements and parame- 
ters. List these at the bottom of the 



Instrument 


Cost of purchase or fabrication 


Dollars 

10 50 100 250 500 1,000 2,000 10,000 
1 , , . 1 . 1 . . 1 . . 1 . . . . 1 ■ 1 .1 


Gloetzl pressure 
cell 


a 


Powered-support 
pressure recorder 




i i 




Prop load eel 1 




i i 




Roof bolt load cell 




i i 




Roof bolt U-cell 


□ 


Surface-mount 
photoelastlCgauge 




i i 




Surface-mount 
strain gauge 




i i 




Torque wrench 




i i 





KEY 

m Cost range 



FIGURE 52. - Cost range of purchase or fabrica- 
tion of support load measuring instruments. 



49 



Instrument 


Range of technical ability required 


Slight 


Moderate 


Extensive 


Gloetzl pressure 
cell 


















pressure recorder 


^^^?\\\V\\\\\\VJ 


i 






Prop load cell 








CV 


wwwwwww 


v\W\\l 


Roof bolt load cell 










vWl 










Roof bolt U-cell 








^V\\\\s\\\\v 


| 


Surface-mount 
photoelastic gouge 


1 




1 






IA^TO^\\\\\\\\V) 


Surface-mount 
strain gauge 












k\\\\\\\\V 






Torque wrench 








^^^^5 





KEY 
D Installation 
I Monitoring 



k\\\\N Data interpretation 

FIGURE 53. • Range of technical ability required 
for installation, monitoring, and data interpretation 
of support load measuring instruments. 



Instrument 


Type of measured dota obtained 


Visual 


Simple numerical 


Detailed numerical 


Gtoetzyl pressure 
cell 




o 




Powered-support 
pressure recorder 




o 


o 


Prop load cell 




o 




Roof bolt load cell 




o 


Roof bolt U-cell 




o 




Surface -mount 
phone nsi c jauge 




o 


o 


Surface -mount 
strain gouge 






o 


"Vque * r ench 


o 


o 





KEY 
O Dota obtainable 



FIGURE 54. - Type of measured data obtained 
from support load measuring instruments. 



worksheet. If no instruments satisfy all 
of the requirements and parameters, it 
may be necessary to change the parameters 
or to choose the instruments that satisfy 
the most requirements and parameters. 

Step 6: At this point, the user must 
select the most suitable instrument(s) , 
based on the instrument descriptions 
following. 

DESCRIPTION OF INDIVIDUAL INSTRUMENTS 

Gloetzl Pressure Cell 

Principle and Application 

Gloetzl pressure cells are used for 
measuring the pressure in tunnel linings, 
rock formations, earth fills, and sup- 
ports. Pressure is measured by a fluid- 
filled diaphragm (J_). 10 

Availability 

The cell is available from Roctest, 
Inc. , or Sinco, Slope Indicator Co. at a 
cost of approximately $2,500. This in- 
cludes the cell, installation equipment, 
and readout gauges. 

Description 

The cell is basically a flat, fluid- 
filled diaphragm made of stainless steel 
or copper. A unique feature of this cell 
is a hydraulic bypass valve, which iso- 
lates the cell fluid from the line fluid, 
resulting in a stiffer and more accurate 
device. 

Installation 

For monitoring pressures in rock: 

Step 1: Drill a series of horizontal 
holes to create a slot in the rock. The 
slot should be at least 8 in wide and 0.5 
in high. 

Step 2: Insert the cell into the slot. 
It will be necessary to use steel plates 
on each side of the cell to make the cell 
the appropriate thickness. 

Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this chapter. 



50 



For monitoring support load: Install 
the cell between the prop and mine roof 
or between the blocks of a crib, then 
wedge the support tight. 

Data Collection 

Oil is pumped into the cell until the 
bypass valve opens. At this point, con- 
tinued pumping produces no further in- 
crease in the input pressure. This by- 
pass pressure is read and is equivalent 
to the external pressure acting on the 
cell or to the load on the support. 

Data Interpretation 

The reading obtained is the pressure 
acting on the cell at the cell location, 
which may not be the true pressure in 
the rock. Many pressure measurements 
taken at various locations should reveal 
a more precise picture of the total rock 
pressure. 

Powered-Support Pressure Recorder 

Principle and Application 

A powered-support pressure recorder is 
a clockwork-operated recorder, connected 
to the hydraulic support systems of 
powered supports (longwall chocks and 
shields or automated temporary roof sup- 
ports (ATRS)) (_2). It measures and rec- 
ords pressure changes that reflect load 
changes on the support. 

Availability 

This instrument is available from Bris- 
tol Division, Acco Industries Inc. , and 
Weksler Instruments Corp. The approxi- 
mate cost of this instrument ranges from 
$1,000 to $2,000. 

Description 

A powered-support pressure recorder 
consists of a clockwork mechanism, pa- 
per chart, ink pen, hydraulic hoses 
and fittings, and protective case (fig. 
55) . It is connected to the hydraulics 
of the support and measures all pres- 
sure changes. The charts are of 1-week 



duration and are either mechanically 
(spring) or electrically driven. 

Installation 

Step 1: Select a mounting location as 
free as possible from dust, grime, vibra- 
tion, and extremes of temperature. 

Step 2: Mount instrument and case as 
securely as possible. 

Step 3: If chart is electrically driv- 
en, wire instrument to voltage shown on 
instrument (120 or 240 V). 

Step 4: Connect hydraulic connections 
on back of case to hydraulics of support 
jacks. Detailed hydraulic hookup in- 
structions can be obtained from instru- 
ment suppliers (3). 

Data Collection 

The pressures are recorded on the 
chart, which can be read as often as de- 
sired and will reveal any substantial 
increase or decrease that occurs (fig. 
56) . The chart should be changed once 
a week, usually on the day when it will 
expire. 




FIGURE 55. - Powered-support pressure recorder. 



51 




* r a i a i 



FIGURE 56. • Chart of hydraulic pressures in 
longwall roof supports during normal operation (4). 

Data Interpretation 

Most installations are designed to mon- 
itor the pressures to see if the support 
system is nearing its yield point. Large 
increases in pressure indicate loading 
from the roof that could lead to in- 
creased difficulty repositioning the sup- 
port. It could also mean that the roof 
could fall when supports are lowered; 
that is , the fall line has overridden the 
supports and is now at the coal face. It 
is important to match the pressure re- 
corded to the corresponding physical 
conditions observed. This will reveal 
which pressures are acceptable, which 
are not acceptable, and what types of ad- 
verse roof conditions and/or support 
problems can be expected from a particu- 
lar pressure. 

Prop Load Cell 

Principle and Application 

A prop load cell is a device that mea- 
sures the load on a roof support prop 
(steel jack, timber, post, crib, rail 
bar, etc.). Prop load cells are inserted 
between the prop and the mine roof. The 
load measured is the result of a roof 



movement (roof sag) and is generally 
attributed to poor pillar design, mine 
design, entry orientation, or support 
capabilities. 

There are three types of prop load 
cells: Multiple strain gauge (electron- 
ic strain readout), hydraulic-pneumatic 
(pound-per-square-inch readout), and pho- 
toelastic (photoelastic readout) . 

Availability 

Prop load cells are available from the 
following suppliers: BLH Electronics; 
Enerpac; Geokon, Inc.; Irad Gage; Roc- 
test, Inc.; Sensotec, Inc.; Sinco, Slope 
Indicator Co.; and Strainsert Co. The 
approximate cost for a load cell and 
readout system ranges from $250 to 
$2,000. 

Description 

Electrical strain gauge readout cells 
are constructed from high-strength steel, 
stainless steel, or titanium, to which 
several sets of matched strain gauges are 
bonded (fig. 57) (5 ) . The cell is con- 
nected to the readout box by an electri- 
cal cable. 

Hydraulic-pneumatic readout cells are 
constructed of two plates welded together 
at their outer circumference and filled 
with antifreeze (6-^7_). They have either 
a pressure gauge or pressure transducer 
readout. 

Photoelastic readout cells consist of 
a high-tensile-steel cylinder that houses 
a cylindrical glass transducer within 
a diametrical hole (fig. 58) (8). The 
glass cylinder is illuminated with a 
polarizer light and viewed through a hand 
viewer. 

Installation 



No special tools or training are needed 
to install prop load cells. They are 
simply inserted between the prop and a 
header board (which is against the mine 
roof) then wedged down in the normal prop 
installation method. Care must be taken 
to install the prop and cell as nearly 
perpendicular to the roof as possible to 
ensure accurate load measuring. Also, 



52 






2 

Scale, srs 





FIGURE 57. - Prop load cell (strain gauge design) (5). 





O 2 

I i I 

Scale, in 



KEY 
/ Prop load cell 
2 Light polarizer 
J Handheld viewer 



FIGURE 58. - Photoelastic prop load cell and 
readout equipment (8). 

all load cells should be calibrated prior 
to installation. 

A reading should be taken immediate- 
ly after installation. This reading 
serves as a reference for all additional 
readings. 

Data Collection 

Readings from an electrical strain 
gauge cell are taken by connecting the 



cell to the readout box, using an elec- 
trical cable. The reading will appear on 
the screen of the readout box. 

Readings from the hydraulic-pneumatic 
cell are taken either by reading the dial 
gauge, or for some models, by connecting 
the pressure transducer between the cell 
and readout box. 

Readings for the photoelastic cell 
are slightly more difficult. First, the 
polarizer is inserted into the opposite 
side of the cell. The handle of the 
handheld viewer is held in line with the 
loading axis of the cell. The aperture 
on the viewer has a rotating compensation 
scale. With the scale initially set at 
zero, the number of visible photoelastic 
fringes (see fig. 50) in one-half of 
the glass cylinder is counted. An exact 
count is made when the fringe at the 
center forms a cross (X). Where this is 
not the case, the full fringes are count- 
ed, and then the compensation scale is 
rotated clockwise until the last fringe 
counted (nearest the center) has moved 
back to form a cross. In this case, the 
scale reading is added to the initial 



53 



full fringe count. The fringe count is 
multiplied by the appropriate cell sensi- 
tivity factor to obtain the load (9_) . 

This instrument should be read once a 
week. However, the frequency of readings 
depends on roof conditions and the amount 
of load detected. Large load changes or 
visible roof changes require frequent 
readings . 

Data Interpretation 

Each mine must establish how much load 
is acceptable. The strength of the props 
can be determined that corresponds to 
the maximum load they can support. 
When the maximum load is reached, the 
props, and possibly the roof, will fail. 
Therefore, additional props should be 
installed before this maximum load is 
reached. 

Roof Bolt Load Cell 

Principle and Application 

A roof bolt load cell is designed to 
measure the load on mechanical anchor 
bolts. It has a hole through the center 
so that it can be slid onto a roof bolt. 
Final position is between the roof and 
bolt plate. If the roof is uneven, it 
may be necessary to put a steel plate be- 
tween the load cell and roof. 

A roof bolt load cell measures load 
by one of three methods: (1) sets of 
matched strain gauges within the load 
cell, (2) springs and two metal disks, or 
(3) a rubber pad compressed between two 
metal plates. 



Description 

The electronic readout load cells are 
constructed of steel to which several 
sets of matched strain gauges are bonded 
(fig. 59) (_5_ ) . Load on the bolt is mea- 
sured as a change in strain, as measured 
by the strain gauges. These cells are 
read by a strain indicator box connected 
to the cell by an electrical cable. 

The spring-and-disk load cell measures 
the load by a change in distance between 
the two disks (fig. 60). The springs de- 
termine the load-detecting ability of the 
cell. A depth-measuring dial gauge is 
used to take readings. 

The rubber compression pad cell mea- 
sures the load by a change in thickness 
of the pad as measured by the distance 
between the two metal plates. 

Installation 

Step 1: Drill a standard roof bolt 
hole at the desired location. 

Step 2: Remove anchor assembly from a 
roof bolt. 

Step 3: Slide roof bolt plate onto the 
bolt, followed by the load cell. 

Step 4: Replace anchor assembly on the 
bolt. Take an initial, unloaded measure- 
ment of the cell. 

Step 5: Install the bolt in the hole, 
and tighten to torque specified by mine 
roof control plan. Take care to install 
the load cell perpendicular to the axis 
of the roof bolt. 

Step 6: Take an initial reading to 
serve as the reference for subsequent 
readings. 



Availability 



Data Collection 



Roof bolt load cells can be purchased 
from the following suppliers: Ailtech; 
Geokon, Inc.; Goodyear Tire and Rubber 
Co.; Irad Gage; Roctest, Inc.; Sensotec, 
Inc.; Sinco, Slope Indicator Co.; and 
Strainsert Co. The approximate cost of 
the cell and readout system ranges from 
S150 to $2,000. 



When first installed, these load cells 
should be read each day until the read- 
ings have stabilized (when little or no 
change occurs). The frequency should 
then be decreased to once a week. If the 
readings do not stabilize, daily readings 
should continue until stabilization oc- 
curs or unsafe roof conditions develop. 



54 






FIGURE 59. - Typical roof bolt load cells 
(strain gauge design) (5). 



Sensor plate for 
readout 



Central hole for rock bolt, 
in diameter 




Xup springs, 16 tons Load distribution plate 

FIGURE 60. - Cross-sectional view of a spring- 
and-disk roof bolt load cell (]_0). 

Data Interpretation 

The suppliers of load cells will 
furnish the information (cell spring 
rate, deflection analysis, graphs, 
charts, equations, etc.) needed to con- 
vert the readings (strain or inches of 
deflection) to the actual load on the 
bolt. Bolt load is needed to determine 
if bolt failure or anchorage failure 
will occur. The amount of load that 
will cause these failures depends on 
the type of bolt, the geology of the 
roof strata, the yield and barrier 
pillar dimensions , and the amount of 
overburden. 



Roof Bolt U-Cell 

Principle and Application 

Roof bolt U-cells are designed to mea- 
sure the load on roof bolts due to roof 
sag, strata separation, or continued area 
mining (11) . They can also measure ap- 
plied bolt load when a bolt is torqued. 
U-cells do not allow mine personnel to 
identify the origin of the load exactly, 
but the measurements are used for design 
modification or to determine support load 
capability. 

Availability 

U-cells can be purchased or fabricated 
in-house. U-cells can be purchased from 
Sinco, Slope Indicator Co., for approxi- 
mately $250 each. At present, the in- 
house fabrication cost is approximately 
$175 each, including labor and material. 

Description 

U-cells are U-shaped, thin-walled, 
fluid-filled metal bladders with an at- 
tached pressure gauge (6). They are 
constructed from copper pipe and tubing 
with brass fittings. They are sand- 
wiched between steel plates, held to- 
gether by split-ring keys, to ensure uni- 
form pressure on the cell during loading 
(fig. 61). 

Installation 

U-cells are installed on pattern bolts 
during the mining cycle or on additional 
bolts in previously mined workings. 

Step 1: Drill a standard roof bolt 
hole at the installation site. 

Step 2: Insert the roof bolt into the 
hole until only 3 in protrudes. 

Step 3: Slide the U-cell between roof 
and bolt plate with the bolt in U part of 
cell. Be sure the gauge is facing the 
entry to facilitate data collection. 

Step 4: Push the bolt against the roof 
and tighten to torque specified in mine 
roof control plan. 

Step 5: Take the first reading immedi- 
ately after installation to serve as a 
reference for all additional readings. 



55 




Scole, in 



FIGURE 61. • Steps in fabrication of a roof bolt 
U-cell. .4, Copper tube cut to length and flattened 
to 1 4 in; B, top half of ends cut; C, top half of 
ends removed; D, bottom of ends folded up and 
brazed to top; E, cells drilled and ready for con- 
nection of gauge, crossover tubing, and relief valve; 
F, completed cell. 

Data Collection 

U-cells should be read once a week, 
more often if large changes in pressure 
occur. 

Data Interpretation 

The readout data, in pounds per square 
inch, must be converted to pounds of load 
before they are useful for determining 
support load capabilities. The equation 

Gauge reading (psi) 

x area of U-cell (in 2 ) 

= bolt load (lb) 

converts the gauge reading to bolt load, 
which when compared with the bolt speci- 
fications, will indicate when the bolt 
could fail. 



Surface-Mount Photoelastic Gauge 

See chapter 4 for a complete descrip- 
tion of this instrument. 

Surface-Mount Strain Gauge 

See chapter 4 for a complete descrip- 
tion of this instrument. 

Torque Wrench 



Principle and Application 



A torque wrench is a 
that measures the torque 
an installed roof bolt, 
marily to test mechani 
immediately after inst 
measurements are useful 
the effectiveness of a 
the structural integrity 
anchorage conditions (12 

Availability 



mechanical gauge 

(foot pounds) on 

It is used pri- 

cal anchor bolts 

allation. Torque 

for determining 

bolting pattern, 

of the bolt, and 

). 



Torque wrenches are available from tool 
manufacturers and from suppliers of mine 
hand tools and safety equipment. A 
partial list of torque wrench suppliers 
includes Armstrong Bros. Tool Co.; Klein 
Tools, Inc.; and Snap-on Tools Corp. 
The approximate cost of a torque wrench 
ranges from $150 to $350. 

Description 

The system consists of a long-handled 
wrench, dial gauge, coupler mechanism, 
and socket that fits on the roof bolt 
head (fig. 62). Most dial gauges have 
double pointers, one of which stays at 
the maximum reading until reset. 

Installation 



No installation is required. Before 
using the wrench, set both dial gauge 
pointers to zero by rotating the dial or 
pointers. 



56 




FIGURE 62. - Torque wrench. 



Data Collection 

Step 1: Fit socket and wrench onto the 
bolthead. 

Step 2: Apply constant pressure to the 
bolt until it just moves. 

Step 3: Remove the wrench from the 
bolt. Read the dial gauge and record the 
reading. 

Step 4: If a second reading is de- 
sired, reset the gauge to zero, then fol- 
low steps 1 through 3. 

Federal regulations require that 10 
pet of all bolts inby the last open 
cross-cut be torque tested each shift. 
This reveals unfavorable bolt conditions 
relatively soon after installation. 
Roof bolts outby can also be checked if 
necessary. 

Data Interpretation 

Data are limited to the loss or gain 
of torque by the roof bolts. Torque 
loss can result from bad bolts, improp- 
er installation, or anchorage failure. 
Torque gain indicates loading of the bolt 
by the roof. 

Torque in foot pounds (T) can be ex- 
pressed as pounds of tension (P) by the 
following equation: 

P = 42. 5T - 1,000. 

P will be within 2,700 lb of the torque 
value predicted by this equation 90 
pet of the time. For example, a torque 
measurement of 260 fflbf indicates a 



bolt load of 10,000 ±2,700 lb. For a 
quick approximation, 1 ft»lbf torque cor- 
responds to a bolt load of 40 lb ( 12 ) . 
The above equation is general and may 
need to be modified if the bolts are in- 
stalled with hardened washers and lubri- 
cated threads. 

REFERENCES 

1. Terrametrics (Golden, CO). Gloetzl 
Pressure Cell. Bull. A-0897, 1973, 
2 pp. 

2. Shepherd, R. , and D. P. Ashwin. 
Measurement and Interpretation of Strata 
Behaviour on Mechanized Faces. Colliery 
Guardian, v. 216, No. 12, 1968, pp. 795- 
800. 

3. Bristol Division, Acco Industries 
Inc. (Waterbury, CT) . Indicating and 
Recording Pressure Gauges. Brochure 
Y19800, 1973, 40 pp. 

4. Moebs, N. N., and E. A. Curth. 
Geologic and Ground-Control Aspects of 
an Experimental Shortwall Operation in 
the Upper Ohio Valley. BuMines RI 8112, 
1976, 30 pp. 

5. Terrametrics (Golden, CO). Load 
Cells. Bull. A-0899, 1973, 2 pp. 

6. Wang, C. Survey of Tools and Tech- 
niques for Roof Control Studies in Under- 
ground Coal Mines. BuMines, PMSRC Inter- 
im Report, Mar. 1972, 43 pp.; available 
upon request from E. R. Bauer, Bureau of 
Mines, Pittsburgh, PA. 

7. Sinco, Slope Indicator Co. (Seat- 
tle, WA). Total Pressure Cell, Model 
51482. Brochure, 1977, 2 pp. 



8. Terrametrics (Golden, CO). Photo- 
elastic Prop Load Cell. Bull. 21.21, 
1968, 2 pp. 

9. Strainsert Co. (West Conshohocken, 
PA). Load Cells. Brochures 365-4A, 368- 
1, 365-4MP, 101B, 104, 107, Oct. 1979. 

10. Roctest, Inc. (Plattsburgh, NY). 
Rock Bolt Load Cell, Model D-16. Bro- 
chure, 1979, 2 pp. 



57 



11. Terrametrics (Golden, CO). Hy- 
draulic Load Cells. Bull. A-8095, 1973, 
2 pp. 

12. Chugh, Y. P. Practical Roof Con- 
trol in Coal Mining. A Short Course 
(South. IL Univ. , Carbondale, IL, Dec. 
1977). Available upon request from Y. P. 
Chugh, South. IL Univ. , Carbondale, IL. 



DISCUSSION 



This manual is intended not as an ex- 
haustive instrument catalog but as a 
basic guide for selecting ground movement 
measuring instruments. Additional infor- 
mation can be obtained from the instru- 
ment suppliers and/or the author of this 
guidebook. 

A ground control program can be devel- 
oped in two ways. The first is the un- 
limited dollars and unlimited personnel 
approach. The second works with limited 
dollars and limited personnel. It is 
generally felt that the second approach 
is better for a company that is new to 
ground control instrumentation. The ap- 
pointment of one or two mine engineers as 
ground control specialists to conduct 
tests, evaluate data, etc., will result 
in instrumentation programs that are bet- 
ter managed and more cost effective, and 
produce more reliable (useful) data and 
solutions. 



Mine operators may encounter certain 
problems when introducing ground control 
instrumentation. One is that accidental 
or deliberate damage to the instrument 
can result in false and inaccurate read- 
ings. Another is that workers may mis- 
interpret instrument outputs and be 
reluctant to work in an area they in- 
terpret to be unsafe. These problems 
can be largely eliminated by explaining 
to all employees (1) the reasons for 
instrumenting the area, (2) the extent 
to which movement actually indicates 
unstable conditions, and (3) the added 
margin of safety that such instrumenta- 
tion gives them. Labor as well as man- 
agement must realize that an efficient 
ground control program is the corner- 
stone to a safe and productive work 
environment. 



58 



BIBLIOGRAPHY 1 1 



Aggson, J. R. How To Plan Ground Con- 
trol. Coal Min. & Process., v. 16, No. 
12, 1979, pp. 70-73. 

Armstrong Bros. Tool Co. (Chicago, 
IL). Armstrong Tools. Catalog 880A, 
1980, pp. 92-94. 

Barnes Engineering Co. (Stamford, CT). 
Instatherm. Bull. 14-220A, 1978, 4 pp. 

Barry, A. J. Ground Control With Long- 
wall Mining. Min. Congr. J. , v. 56, No. 

6, 1970, pp. 53-55. 

Barry, A. J., and 0. B. Nair. In Situ 
Tests of Bearing Capacity of Roof and 
Floor in Selected Bituminous Coal Mines. 
BuMines RI 7406, 1970, 20 pp. 

Barry, A. J., and J. J. Wojciechow- 
ski. Roof-Movement Study of Mechanized 
Retreating Longwall Operation, Lancashire 
No. 15 Mine, Bakerton, Cambria County, 
Pa. BuMines RI 5028, 1954, 13 pp. 

BLH Electronics (Waltham, MA). Pres- 
sure, Strain, Load Measurement. Bull. 
001, 1980, 46 pp. 

Bristol Division, Acco Industries Inc. 
(Waterbury, CT). Bristol Recording Pres- 
sure Gauges. Bull. G621A, 1970, 19 pp. 

Caudle, R. D. Mine Roof Stability. 
Paper in Ground Control Aspects of Coal 
Mine Design. Proceedings: Bureau of 
Mines Technology Transfer Seminar; Lex- 
ington, Ky.; March 6, 1973, comp. by 
Staff, Mining Research. BuMines IC 8630, 
1974, pp. 79-83. 

Corwine, J. W. Roof Control Is More 
Than Just Protection at the Mine Face. 
Coal Min. & Process., v. 13, No. 11, 
1976, pp. 48-49, 51-52, 71. 

Dejean, M. J. P. Deformation Analysis 
in Underground Roadways. Int. J. Rock 
Mech. and Min. Sci. and Geomech. Abstr. , 
v. 13, 1975, pp. 25-30. 

Deshwar, R. H. S. "E-H" Extensometer: 
A Practical Instrument for Rock Mechanics 
Measurements. Can. Min. J., v. 91, No. 

7, 1970, pp. 45-47. 

Eder Instrument Co. , Inc. (Chicago, 
IL). Eder Borescopes. Brochure 79/5, 
1980, 4 pp. 

11 These publications are listed as 
a source of additional information on 
ground control and instrumentation. 



Enerpac (Butler, WI). Hydraulic Tools 
for General Construction. Brochure 
CS652, 1979, p. 37. 

Extech International Corp. (Boston, 
MA). Portable Meters and Probes. Cata- 
log 1979-80, 1980, 17 pp. 

Franklin, J. A. Rock Mechanics Review. 
The Monitoring of Structures in Rock. 
Int. J. Rock Mech. and Min. Sci. and 
Geomech. Abstr. , v. 14, 1977, pp. 163- 
192. 

Hooker, V. E. , J. R. Aggson, and D. L. 
Bickel. Improvements in the Three- 
Component Borehole Deformation Gage and 
Overcoring Techniques. BuMines RI 7894, 
1974, 29 pp. 

Hooker, V. E., and D. L. Bickel. Over- 
coring Equipment and Techniques Used in 
Rock Stress Determination. BuMines IC 
8618, 1974, 32 pp. 

Industrial Products Co. (Chicago, IL). 
Industrial Products. Catalog, 1980, p. 
63. 

Klein Tools, Inc. (Chicago, IL). Klein 
Tools. Catalog 123, 1980, p. 24. 

Kulite Semiconductor Products, Inc. 
(Ridgefield, NJ). Solid State Transducer 
Technology. Bull. SF-l-B, 1980, 3 pp. 

Leeman, E. R. Remote Measurement of 
Rock Stress Under Development in South 
Africa. Eng. and Min. J., v. 165, No. 9, 
1964, pp. 104-107. 

Microdot, Inc. (Greenwich, CT). Welda- 
ble Strain Gages. Catalog of instrumen- 
tation products, 1971, 61 pp. 

Mikron Instrument Co. (Ridgewood, NJ). 
Infrared Thermometers. Brochure, 1980, 
4 pp. 

Olsen, A. J., J. M. Bryant, M. J. Pend- 
er, and P. E. Salt. Instrumentation for 
Tunnelling. Paper in Tunnelling in New 
Zealand (Proc. Hamilton Tech. Group). 
N.Z. Inst. Eng., Wellington, N.Z., v. 3, 
1977, pp. 5.14-5.25. 

Olympus Corp. of America (New Hyde 
Park, NY). Olympus Focusing Borescopes. 
Brochure SM-380 5R, 1980, 5 pp. 

Panek, L. A. Measurement of Rock 
Pressure With a Hydraulic Cell. Min. 
Eng. (N.Y.), v. 13, No. 3, 1961, pp. 282- 
285. 



59 



Parker, J. How To Design Better Mine 
Openings. Eng. and Min. J. , v. 174, No. 
12, 1973, pp. 76-80. 

Peng, S. S. Coal Mine Ground Control. 
Wiley, 1978, 450 pp. 

Poad, M. E., G. G. Waddel, and E. L. 
Phillips. Single-Entry Development for 
Longwall Mining. BuMines RI 8252, 1977, 
29 pp. 

Rana, M. H. Rock Mechanics and the 
Mining Industry. Can. Min. and Metall. 
Bull., v. 59, No. 654, 1966, pp. 1177- 
1183. 

Raytek Inc. (Mountain View, CA). 
Raynger II Infrared Thermometer. Bro- 
chure, 1980, 2 pp. 

. Raytek Raynger, Brochure C100, 

1980, 4 pp. 

. Raytek Raynger, Brochure RA- 



111/5M, 1979, 6 pp. 

Reed, J. J. Recent Developments in 
Rock Engineering. Min. Congr. J. , v. 55, 
No. 5, 1969, pp. 29-30. 

Sensotec, Inc. (Columbus, OH). Strain 
Gage Load Cells. Brochure, 1979, 12 pp. 

Shepherd, R. Strata Control: A Review 
of Research at the Mining Research Estab- 
lishment, Isleworth. Colliery Guardian, 
v. 217, No. 8, 1969, pp. 450-454. 

Singhal, R. K. Fourth International 
Conference on Strata Control and Rock 
Mechanics. J. Mines, Met., and Fuels, 
v. 12, No. 10, 1964, pp. 310-311. 



Skilton, D. Determination of Stress 
Variations in Coal. Min. Mag., v. 118, 
No. 2, 1968, pp. 105-107. 

Sinco, Slope Indicator Co. (Seattle, 
WA). Digitilt Model 50306. Brochure 1- 
76, 1976, 2 pp. 

. Digitilt Tiltmeter Model 50322. 

Brochure 7-75, 1975, 1975, 2 pp. 

Snap-on-Tools Corp. (Kenosha, WI). 
Snap-on Tools. Catalog, 1980, pp. 128- 
135. 

. Snap-on Torque Wrenches. Bro- 
chure SS 477C 10M-B, 1974, 19 pp. 

Terrametrics (Golden, CO). Recording 
Convergence Meter. Brochure A-0900, 
1973, 2 pp. 

. Tape Extensometer. Brochure 

A-0894, 1973, 2 pp. 

Walrod, G. , and L. Adler. Analyzing 
Development Roof Falls. Coal Age, v. 76, 
No. 3, 1971, pp. 106-110. 

Weksler Instruments Corp. (Freeport, 
NY). Installation, Operation and Service 
Instructions. Bull. M 19-42, 1977, 8 pp. 

Welch Allyn, Inc. (Skaneateles Falls, 
NY). Welch Allyn. Bull. FB 3-77, 1980, 

6 pp. 

Wilson, A. H. The Measurement of Rock 
Stress. Colliery Eng. (London), v. 24, 
No. 5258, 1962, pp. 118-120, 125-127. 

Woodruff, S. D. Practical Use of Rock 
Mechanics. Min. Congr. J., v. 47, No. 7, 
1961, pp. 37, 55. 



60 



APPENDIX A. —CASE STUDIES 



Case Study 1: Convergence Stations and 
Dial Gauge Tube Extensometer (O 1 

This study was conducted by Bureau 
personnel and mine engineers at an under- 
ground coal mine in southwestern Pennsyl- 
vania. It involved measuring the magni- 
tude and rate of advance of a squeeze. 
The squeeze, starting as a slow increase 
in weight on the pillars and continuing 
to total entry closure, was probably due 
to leaving undersized pillars to support 
the main roof. 

Fifteen pairs of convergence stations 
were installed in the roof and floor out- 
by the advancing squeeze. Measurements 
from these stations would reveal acceler- 
ated squeeze movement. Early warning was 
desired so that barrier support struc- 
tures could be built to stop the squeeze. 
A tube extensometer with a dial gauge 
readout was used to make initial conver- 
gence measurements at the time of station 
installation, as well as subsequent mea- 
surements. At first, measurements were 
made once a week, but since convergence 
was slight, the time interval was in- 
creased to 2 weeks. 

Convergence at each station was graphed 
to show its movement with respect to 
time, and contour maps of total conver- 
gence for each station were drawn to show 
the direction of squeeze movement. Maxi- 
mum convergence for the first 8 months 
was 1.889 in; average convergence was 
0.397 in. Underground observations just 
inby the stations revealed squeezing of 
cribs, floor heave, rib spalling, and 
small, scattered roof falls. 

The analysis of the convergence data 
through maps , graphs , and underground ob- 
servations indicated that after 6 months 
the squeeze had decelerated significant- 
ly, owing to a large block of solid coal 
that had been left to protect a gas well. 
Measurements were continued, to serve 
as an alarm in case of further squeeze 
movement. 

Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this appendix. 



Case Study 2; Simple Weighted Bed 
Separation Indicators, Horizontal Roof 
Strain Indicators, and Stratascope (2^) 

This investigation was conducted by 
Bureau personnel at an underground coal 
mine near Somerset, CO. Various instru- 
ments were installed in the roof of a 
room-and-pillar section in an attempt to 
determine the effects of length of time 
between exposure and support on mine roof 
stability. Simple weighted bed separa- 
tion indicators were installed to measure 
differential roof displacement during and 
after mining. Horizontal roof strain in- 
dicators (HORSI) were used to determine 
the existence of horizontal stress fields 
that might influence roof stability. In 
addition, a drill hole was provided at 
each location for use with a stratascope 
to determine the locations and widths of 
roof separations. 

Typical instrumentation stations con- 
sisted of three simple weighted bed sep- 
aration indicators, one or two horizontal 
strain indicators, and a stratascope hole 
(fig. A-l). A total of 21 gauge stations 
were instrumented. A hole depth of 7 ft 
was used for the bed separation indica- 
tors. This depth was determined by the 
maximum heights of roof falls near the 
test area. The spring clip anchors were 
placed at 84, 54, 36, and 18 in into the 
roof. The first bed separation indicator 
was installed immediately after bolting 
of the first cut mined. The remaining 
bed separation indicators and the hori- 
zontal strain indicator were installed 
after the next cut was bolted, but with 
varying time intervals between mining and 
supporting. 

Readings were taken daily over the 
first 5 weeks, then intermittently for 
approximately 3 months. Values of to- 
tal change in vertical roof displacement 
ranged from 0.023 to 1.325 in, with an 
average maximum deflection of 0.257 in. 
Horizontal strain indicators showed 
changes of from 5 to 125 yin (1 yin 
equals 1 x 10~ 6 in). The average value 
was 84 y in/in of roof surface. Because 
of these low values, it was determined 



61 





i_ 



Scale, ft 



Coal 



Simple weighted bed 
.separation indicators 




Coal 



HORSI gauge 

Simple weighted bed 
separation indicator 

—-by- 

Stratascope J 
hole 



Direction of mining 

FIGURE A-l. - Typical gauge station. Dashed line 
indicates face location when first gauge was installed. 

that horizontal movement was too small to 
influence roof stability in the test 
area. 

Analysis of displacement values mea- 
sured by the different gauges yielded a 
similarity in overall trends. In gener- 
al, the displacement per unit time (rate 
of sag) was high immediately after mining 
and before bolting took place. After 
bolting operations, the rate of sag fell 
to a much lower value and eventually 
reached a stabilized rate of very little 
or no increase in displacement. As ex- 
pected, the average amount of total dis- 
placement during unsupported time was 
greater for the longer time lapses. The 
rate of sag leveled off to essentially 
zero in all time lapse groups within 10 
to 20 days after full support (fig. A-2). 



KEY 
d Rib gauges 
o Center gauges 
^ Face gauges 




20 40 60 

TIME, days after full support 

FIGURE A-2. - Average deflection, by location. 

The largest differential sag measured 
was between the 18- and 36-in roof lev- 
els. Stratascope observations indicated 
that differential roof movements were 
consistent with gauge readings. A hair- 
line fracture was noted at the 33-in lev- 
el, and a larger separation of 0.06 to 
0.25 in was observed at the 19-in level. 

Case Study 3: Roof Bolt U-Cells, Vibrat- 
ing Wire Stressmeters , Tube Extensometer 
Convergence Stations, Multipoint Bore- 
hole Extensometer Stations (3_) 

This investigation was conducted by 
the Bureau at an underground coal mine 
in southeastern Ohio. Various ground 
control instruments were used to mea- 
sure pillar stresses, bolt loadings, and 
strata separation movements of two long- 
wall gateroads.2 Measurements were made 
by means of roof bolt U-cells, vibrating 
wire stressmeters, convergence stations 
(dial-gauge tube extensometer) , and mul- 
tipoint borehole extensometer stations 
(fig. A-3) . The purpose was to gather 
data on gateroad loadings during develop- 
ment and longwall panel mining in order 
to determine the optimum design of gate- 
road pillars and entries, and artificial 
support requirements. 

Four groups of twelve roof bolt U-cells 
installed in the crosscuts and entries 
measured the change in roof bolt loads 
as different longwall loading situations 
occurred. 

^At the time of publication, very few 
data were available, owing to recent in- 
stallation of instruments. 



62 



Double row V p#P#tt fUTpfHTtTntfn ttp-tt 
cribbed entry p-etftfp-p $$$& pun tin gud- 



Ao 
Z? 


/ 


< 
2 


►J 
► 5 


5 


7 


i 


• 


# 



Belt entry £ 





i_ 








LEGEND 




• 


Vibrating wire stressmeter 


Ao 


D 


Roof bolt U-cell group 


B 


o 


Convergence station 




A 


Multipoint borehole 
extensometer station 



m 



60 



Scale, ft 
FIGURE A-3. - Instrumentation plan for each array. 



Vibrating wire stressmeters installed 
in the abutment and yield pillars mea- 
sured vertical, uniaxial stress changes. 
They were installed at varying distances 
into the pillars. This positioning will 
reveal the general stress distributions 
both lengthwise and widthwise across the 
pillars. Stress differences between the 
core and outer layer of the pillars will 
also be detected. 

Convergence stations have been in- 
stalled so that a dial-gauge tube 
extensometer can be used to measure 
roof-to-floor convergence. Multipoint 
extensometer stations, employing a sonic 
readout borehole extensometer, measure 
roof strata separations. The anchors 
were placed at the 2.5-, 4-, 5.5-, 7.5-, 
and 11-ft levels in the roof. These 
locations were just below the interfaces 
between the various roof rock layers. 
Floor heave is determined by compar- 
ing the measurements from these two 
instruments. 



Case Study 4: Plumb Bobs 

This study was conducted by the Bureau 
at an underground coal mine located 
in south-central Pennsylvania. The mine 
roof was monitored to determine the di- 
rection and amount of lateral movement. 
Plumb bobs were used to measure 
movements . 

Lateral roof movement was suspected for 
the following reasons: (1) seam dips 
averaging 10°, (2) deteriorating roof 
conditions beginning updip and progress- 
ing downdip, (3) shearing of roof bolts, 
and (4) empty roof bolt holes with visi- 
ble offsets. The general opinion of the 
mine employees was that all movement was 
downdip. In the attempt to prove or dis- 
prove this, six areas were monitored with 
plumb bobs. The plumb bobs were located 
in a main travelway that had shown only 
slight indications of movement or adverse 
roof conditions. Observations were made 
weekly to determine if the roof and floor 



63 



were experiencing differential movement. 
No distinct direction of movement was 
found, primarily because of the lack of 
precision of the plumb bob measurements. 

To supplement these findings, approxi- 
mately 40 empty roof bolt holes were in- 
vestigated. Nearly all showed a differ- 
ent direction and amount of movement. 
This movement occurred at the same strata 
interface within each hole (fig. A-4) . 

Case Study 5: Borehole Deformation 
Gauge (4_) 

This investigation was conducted by the 
Bureau at an underground lead-zinc mine 
near Bunker, MO. In situ stress was mea- 
sured in one pillar to establish the pil- 
lar loading conditions and to evaluate 
the stability of the pillar. Laboratory 
tests were run on drill cores to deter- 
mine the compressive strength, Young's 
modulus, shear strength, and the angle 
and coefficient of friction of the mine 
rock. 

Tests on 280 NX (2. 125-in-diam) core 
samples collected from 16 locations pro- 
duced the following values: 

Average strength = 11,900±2,554 psi, 
Young's modulus = 9.27±0.98 x 10 6 psi, 
Specific gravity = 2.75. 

In situ rock stress mesurements were 
obtained using a three-component borehole 
deformation gauge. Overcoring was con- 
ducted in a borehole drilled horizontally 
at midheight of the pillar, to more than 
half the width of the pillar. Owing to 
poor core recovery during overcoring, 
only two sets of usable borehole deforma- 
tion data were obtained. Secondary prin- 
cipal stresses in the plane normal to the 
borehole were calculated to be -10,048 
psi for the vertical stress and -3,701 
psi for the horizontal stress (negative 
sign denotes compressive stresses). 

The measured vertical stress of -10,048 
psi in the pillar is within the average 
compressive strength of 11,900±2,554 psi. 
This indicates that continual spalling 
and pillar deterioration may occur, which 
will increase the vertical stress in the 
pillar, resulting in more pillar deteri- 
oration. Since the pillar stress is 



1.0 in 
N65°W 



1.375 in X I. 375 in 
S85°W S45°W 



h 



1.25 in I 25 in 

S65°E S60°W 



Coal 
pillar j 



0.875 in 
S65°W 



1.0 in 

S70°W 



l- 4 _ 3 Z 5 J n 1. 125 in 



S75°E 



S25°E 




Scale, ft 



FIGURE A-4. - Amount and direction of lateral 
roof movement found in holes 20 through 29. 



nearly equal to 
strength of the r 
strength of the p 
the vertical st 
exists that the p 
exceeded the pill 
pillar is in a 
method of pillar 
warranted in this 



the average compressive 

ock samples tested, the 

illar may be exceeded by 

ress. The possibility 

illar stress has already 

ar strength and that the 

postfailure state. A 

stabilization would be 

situation. 



Case Study 6: Flat Borehole Pressure 
Cells and Powered-Support Pressure 
Recorders (5) 

This study was conducted by the Bureau 
at a mine located in the Ohio Valley area 
of southwestern Pennsylvania. Experimen- 
tal shortwall sections were monitored by 
measuring the relative pressure changes 
in chain pillars and shortwall panels 
with flat borehole pressure cells (encap- 
sulated type) to determine the effect of 
mining, and by recording hydraulic pres- 
sure on some roof support units to deter- 
mine whether or not they approached the 
yield pressure. Chain pillars were moni- 
tored because of the concern that in- 
adequate caving of the limestone roof 



64 



would shift excessive load to the chain 
pillars. Pressure cells were installed 
in selected chain pillars and in short- 
wall panels to determine pressure changes 
due to increased loading, although the 
relationship between the relative pres- 
sures measured and the actual load or 
pressures was not fully understood (fig. 
A-5) . The cells were installed from 4 to 
14 ft deep, oriented to indicate vertical 
loading only, and were initially pressur- 
ized to about 800 psi (the estimated mean 
load exerted by the overburden) . Mea- 
sured pressure changes revealed no large 
increases in vertical pressures in any of 
the chain pillars during or after panel 
extraction (fig. A-6) . The entries were 
generally stable, and there was a notable 
absence of pillar sloughing. 

In addition to measuring relative pres- 
sure changes in the coal, recorders were 
connected into the hydraulic system of 
the powered roof supports to monitor flu- 
id pressures in the front and rear leg 
assemblies. The purpose was to detect 
pressure surges under adverse roof condi- 
tions and to warn of excessive loading on 
the chocks. The recorders were placed on 
chocks near the center and near the tail 
end of the chock line. Both of these lo- 
cations yielded recorder charts indicat- 
ing similar pressure characteristics. 



During normal operation, potential roof 
falls are indicated by slow increases in 
pressure until the failure occurs, imme- 
diately followed by a substantial pres- 
sure jump. None of the chocks monitored 
reached more than 82 pet of its yield 
pressure. 

Case Study 7: Roof Bolt U-Cells and 
Hydraulic Prop Load Cells (6) 

This study was conducted by the Bureau 
at a central Pennsylvania coal mine. The 
mine roof, ribs, and bolts were monitored 
to determine the roof movement and pres- 
sures generated during the formation of 
cutter roof failure. U-cells were used 
to detect bolt loadings while flat, hy- 
draulic prop load cells were used to mon- 
itor loading of fiber cribs. 

Measured bolt loadings indicated that 
the roof was acting as a cantilever beam 
with the most sag on the side where the 
cutter was forming. Bolts near the cut- 
ter had greater load increases (500 to 
600 psi) than those distant from the cut- 
ter (50 to 100 psi). 

Measurements of crib loadings revealed 
that the cribs were able to support a 
considerable amount of fractured roof 
without failing. The average load in- 
crease was 1,000 psi. 




i 1 — 

J Coreholes I and 2 

Dl/z_ju>yj 

J USrehole 

1u 



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QQQQQ QQDO 




borehole 



,16 



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roanaanaoo 

innnn nn nod 
fan 

Surface/ 



2a 




o 

i_ 



300 
i 



LEGEND 
Clay vein 

Flat borehole pressure cell 
Roof fall 
FIGURE A-5. - Shortwall section 7 right. 



Scale, ft 



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65 



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i i i i i — i — i — i — i—i — i — I i i i i — i — r~ i — i i i i i i i i i — i — j i i i i — i I i i i — n — I — i— i — i — i i — |— i — |— i — i — i — I — n — i — r— r 

W/,i'/A Y//A//A V/AV/A Y7^\ 

I2,l3>=r^ 18,19 ^ j 



.11 «£ 



C 



8,9 



6,7 



4,5 



16, 17 x-j 



28,29 



24,25 — - 
(Panel 5) 



34,35 



h 20,21 



30,31 *= 



22,23 



26,27- 



1,2 — 

(Panel 3) 



,1,1, 



14,15 



32,33 \s^ 



KEY 

" — • Pressure cell 

E2ZJ Mining time of panel 



j i i i i L_i i i i i I i i i i i I i i i_i i l_i i i i_l I l_l i_i I Lj i_l i l I i_i l i i_ 



Nov Dec Jan Feb Mar Apr May June July Aug 

1973 1974 

DATE OF MEASUREMENT 

FIGURE A-6. - Pressure changes recorded in pillars and shortwall panels during mining. 

REFERENCES 



1. Bauer, E. R. , and G. J. Chekan. 
Convergence Measurements for Squeeze Mon- 
itoring: Instrumentation and Results. 
BuMines TPR 113, 1981, 9 pp. 

2. Radcliffe, D. E., and R. M. State- 
ham. Effects of Time Between Exposure 
and Support on Mine Roof Stability, Bear 
Coal Mine, Somerset, Colo. BuMines RI 
8298, 1978, 13 pp. 

3. Chekan, G. J., and D. R. Babich. 
Investigation of Longwall Gateroad Roof 
Support Characteristics at Powhatan No. 4 
Mine. Instrumentation Plan. BuMines RI 
8628, 1982, 9 pp. 

4. Horino, F. G., and J. R. Aggson. 
Pillar Failure Analysis and In Situ 



Stress Determinations at the Fletcher 
Mine Near Bunker, Mo. BuMines RI 8278, 
1978, 19 pp. 

5. Moebs, N. N. , and E. A. Curth. 
Geologic and Ground-Control Aspects of 
An Experimental Shortwall Operation in 
the Upper Ohio Valley. BuMines RI 8112, 
1976, 30 pp. 

6. Hill, J. L. Ill, and E. R. Bauer. 
An Investigation of the Causes of Cutter 
Roof Failure in a Central Pennsylvania 
Coal Mine: A Case Study. Pres. at 25th 
U.S. Symp. on Rock Mechanics, Evanston, 
IL, June 25-27, 1984, 12 pp.; available 
from J. L. Hill III, BuMines, Pittsburgh, 
PA. 



66 



APPENDIX B.— INSTRUMENT SUPPLIERS 1 



Ailtech 

19535 East Walnut Dr. 

City of Industry, CA 91748 

(213) 965-4911 

American Optical Corp. 
14 Mechanic St. 
Southbridge, MA 01550 
(617) 765-9711 

Armstrong Bros. Tool Co. 
5273 W. Armstrong Ave. 
Chicago, IL 60646 
(312) 763-3333 

Baltimore Instrument Co., Inc, 
4610 Harford Rd. 
Baltimore, MD 21214 
(301) 426-3656 

Barnes Engineering Co. 
30 Commerce Rd. 
Stamford, CT 06902 
(203) 348-5381 

BLH Electronics 
42 Fourth Ave. 
Waltham, MA 02154 
(617) 890-6700 

Bristol Division, Acco 

Industries Inc. 
40 Bristol St. 
Waterbury, CT 06708 
(203) 756-4451 

Budd Co. 

Grant and Franklin Sts. 
Phoenixville, PA 19460 
(717) 935-0225 

Conkle Inc. 
P.O. Box 190 
Paonia, CO 81428 
(303) 527-4848 

Eder Instrument Co., Inc. 
5115 N. Ravenswood Ave. 
Chicago, IL 60640 
(312) 769-1944 



Enerpac 

13000 W. Silver Spring Dr. 

Butler, WI 53007 

(414) 781-6600 

Expanded Optics Co., Inc. 
14102 Willow Lane 
Westminster, CA 92683 
(714) 894-1388 

Extech International Corp. 
114 State St. 
Boston, MA 02109 
(617) 227-7090 



Geokon, Inc. 
7 Central Ave. 
West Lebanon, NH 
(603) 298-5064 



03784 



Geophysical Instrument and 

Supply Co. , Inc. 
4665 Joliet St. 
Denver, CO 80239 
(303) 371-1940 

Glowlarm Rock Fall Warning 

Devices 
P.O. Box 465 
White Pine, MI 49971 

Handy Geotechnical Instruments, 

Inc. 
P.O. Box 1200, Welch Ave. 

Station 
Ames, IA 50010 

Hitec Corp. 

Nardone Industrial Park 
Westford, MA 01886 
(617) 692-4793 

Hughes Aircraft Co. 
Industrial Products Division 
6155 El Camino Real 
Carlsbad, CA 92008 
(714) 438-9191 



^This list may not include all possible suppliers of ground control measuring 
instruments . 



67 



Industrial Products Co. 
7445 North Oak Park Ave. 
P.O. Box 48022 
Chicago, IL 60648 
(312) 647-7855 

Instrument Technology, Inc. 
Box 381, Mainline Dr. 
Westfield, MA 01085 
(413) 562-5132 

Irad Gage 
Etna Rd. 

Lebanon, NH 03766 
(603) 448-4445 

Klein Tools, Inc. 
7200 McCormick Blvd. 
Chicago, IL 60645 
(312) 677-9500 

Kulite Semiconductor Products, Inc, 
1039 Hoyt Ave. 
Ridgefield, NJ 07657 
(201) 945-3000 



Raytek, Inc. 

325 E. Middlefield at Whisraan 
Mountain View, CA 94043 
(415) 961-1650 

Revere Corp. of America 
845 N. Colony Rd . 
Wallingford, CT 06492 
(203) 269-7701 



Roc test, Inc. 
7 Pond St. 
Plattsburgh, NY 
(518) 561-3300 



12901 



Rogers Arms and Machine Co. 
1426 Ute Ave. 
Grand Junction, CO 81501 
(303) 245-3729 

Seco, Standard Equipment Co. 
9240 N. 107th St. 
P.O. Box 23060 
Milwaukee, WI 53224 
(414) 355-9730 



Lenox Instrument Co., Inc. 
Ill E. Luray St. 
Philadelphia, PA 19120 
(215) 324-4543 



Sensotec, Inc. 
1200 Chesapeake Ave. 
Columbus, OH 43212 
(614) 486-7723 



Microdot, Inc. 
475 Steamboat Rd . 
Greenwich, CT 06830 
(203) 661-1200 



Serata Geomechanics , Inc, 
1229 Eighth St. 
Berkeley, CA 94710 
(415) 527-6652 



Micro-Measurements , Measurements 

Group, 
Vishay Intertechnology , Inc. 
P.O. Box 27777 
Raleigh, NC 27611 
(919) 365-3800 

Mikron Instrument Co., Inc. 
P.O. Box 211 
Ridgewood, NJ 07451 
(201) 891-7330 

Olympus Corp. of America 
■evada Drive 
Hyde Park, NY 11040 
(516) 488-3880 



Sinco, Slope Indicator Co, 
3668 Albion Place North 
Seattle, WA 98103 
(206) 633-3073 

Snap-on Tools Corp. 
8030 E. 28th Ave. 
Kenosha, WI 53140 
(414) 654-8681 

Soiltest, Inc. 
2205 Lee Street 
Evanston, IL 60202 
(312) 869-5500 



68 



Spider Inc. 

4001 Gratiot 

St. Louis, MO 63110 

(314) 535-7868 



Weksler Instruments Corp. 
80 Mill Road 
Freeport, NY 11520 
(516) 623-0100 



Strainsert Co. 

100 Union Hill Rd. 

Union Hill Industrial Park 

West Conshohocken, PA 19428 

(215) 825-3310 

Terra Technology Corp. 
3862-T 148th Ave. , NE. 
Redmond, WA 98052 
(206) 883-7300 



Welch Allyn, Inc. 

99 Jordan Rd. 

Skaneateles Falls, NY 13153 

(315) 685-5788 

Westinghouse Electric Corp. 
Industrial and Govt. Tube Div, 
Westinghouse Circle 
Horseheads, NY 14845 
(607) 796-3211 



Wahl Instruments, Inc. 
5750A Hannum Ave. 
Culver City, CA 90230 
(213) 641-6931 



&U.S. CPO: 1985-605-017/20,121 



INT.-BU.O F MIN ES,PGH.,P A. 28119 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



AN EQUAL OPPORTUNITY EMPLOYER 



POSTAGE ANO FEES PAID 

U.S. DEPARTMENT OF THE INTERIOR 

INT-416 



OFFICIAL0USINESS 
PENALTY FOR PRIVATE USE. S300 



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